[Tables] Fix RequestFailedException contract in GetEntityAsync/GetEntityIfExistsAsync when response body is missing or malformed (Azure#58305)

* Initial plan

* Fix ArgumentNullException in GetEntityAsync/GetEntityIfExistsAsync when ContentStream is null

Add null check for response.ContentStream before calling ResponseToDictionary
in GetEntityInternalAsync. When ContentStream is null on a non-404 response,
throw RequestFailedException instead of letting ArgumentNullException bubble up
from JsonDocument.Parse.

Add regression tests and changelog entry.

Fixes Azure#58303

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* Use await in ThrowsAsync assertions in new tests

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* Set ContentStream to empty MemoryStream before throwing RequestFailedException

When response.ContentStream is null, assign an empty MemoryStream before
throwing RequestFailedException(response). This ensures:
1. The exception message includes the HTTP status code and reason phrase
   (context for callers to diagnose the error)
2. GetRawResponse().ContentStream is non-null after catching the exception
   (protects callers from NullReferenceException when reading raw response)

Also update NullContentStreamMockResponse mock to properly propagate the
setter so the regression tests can verify that ContentStream is non-null
on the thrown exception's raw response.

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* Use explicit message in RequestFailedException when ContentStream is null

Replace RequestFailedException(response) with RequestFailedException(status, message)
to avoid the misleading "Service request succeeded." text that was produced
when ContentStream was null on a 200 OK response.

The new message explicitly says "Response body was null or empty. HTTP {status} ({reason})"
which clearly communicates what went wrong and includes the status code for context.

Also simplify NullContentStreamMockResponse back to a read-only getter,
and update test assertions to verify the status code and message content.

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* Apply suggestion from @jsquire

* Update sdk/tables/Azure.Data.Tables/src/TableClient.cs

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* Handle empty/malformed ContentStream: catch JsonException and rethrow as RequestFailedException

The null ContentStream check only covered one scenario. An empty or malformed
(but non-null) stream causes JsonDocument.Parse to throw JsonException, which
would leak an internal parsing exception to callers.

Fix: wrap SerializationHelpers.ResponseToDictionary in a try/catch for
JsonException and rethrow as RequestFailedException with a descriptive message.

Also:
- Add System.Text.Json using for JsonException
- Add 4 regression tests covering empty MemoryStream scenario
- Update CHANGELOG wording to match the null ContentStream scope of the fix

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* Eagerly detect empty ContentStream at the null guard (L797)

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* De-duplicate RequestFailedException throws into a single catch block

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* Restore 'or malformed' in RequestFailedException message

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---------

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Author: 3 people

sdk/tables/Azure.Data.Tables/CHANGELOG.md

@@ -13,6 +13,9 @@ Thank you to our developer community members who helped to make Azure Tables bet
 ### Breaking Changes
 
 ### Bugs Fixed
+- Fixed an error handling issue where `GetEntityAsync` and `GetEntityIfExistsAsync` could throw an
+  `ArgumentNullException` instead of `RequestFailedException` when the HTTP response had a null
+  `ContentStream`. ([#58303](https://github.com/Azure/azure-sdk-for-net/issues/58303))
 - Fixed an `IndexOutOfRangeException` when constructing `TableServiceClient` or `TableClient` with a loopback URI that does not contain an account name in the path, such as the Cosmos DB emulator endpoint `http://localhost:8902/`.
 - Fixed an issue where `TimeSpan` properties in strongly typed table entities were not being deserialized.
 - Fixed an issue when deserializing strongly typed table entities with enum properties. Enum values that aren't defined in the enum type are now skipped during deserialization of the table entity.

sdk/tables/Azure.Data.Tables/src/TableClient.cs

@@ -8,6 +8,7 @@
 using System.Linq;
 using System.Linq.Expressions;
 using System.Net;
+using System.Text.Json;
 using System.Threading;
 using System.Threading.Tasks;
 using Azure.Core;
@@ -792,12 +793,22 @@ await _tableOperations.QueryEntityWithPartitionAndRowKeyAsync(
                 {
                     return new NoValueResponse<T>(response);
                 }
-                else
+
+                Dictionary<string, object> dictionary;
+                try
                 {
-                    var dictionary = SerializationHelpers.ResponseToDictionary(response);
-                    var result = dictionary.ToTableEntity<T>();
-                    return Response.FromValue(result, response);
+                    dictionary = SerializationHelpers.ResponseToDictionary(response);
                 }
+                catch (Exception ex) when (ex is ArgumentNullException or JsonException)
+                {
+                    throw new RequestFailedException(
+                        response.Status,
+                        $"The response body was unexpectedly missing or malformed, so the entity could not be read from the response. HTTP {response.Status} ({response.ReasonPhrase}).",
+                        ex);
+                }
+
+                var result = dictionary.ToTableEntity<T>();
+                return Response.FromValue(result, response);
             }
             catch (Exception ex)
             {

sdk/tables/Azure.Data.Tables/tests/TableClientTests.cs

@@ -785,6 +785,78 @@ public async Task GetEntityIfExistsAsyncDoesNotThrowObjectDisposedException()
             Assert.AreEqual("world", result.Value.GetString("Value"));
         }
 
+        /// <summary>
+        /// Regression test for https://github.com/Azure/azure-sdk-for-net/issues/58303.
+        /// Verifies that GetEntityIfExistsAsync throws RequestFailedException instead of
+        /// ArgumentNullException when the HTTP response has a null ContentStream, and that
+        /// the exception message includes the HTTP status code.
+        /// </summary>
+        [Test]
+        public async Task GetEntityIfExistsAsyncThrowsRequestFailedExceptionWhenContentStreamIsNull()
+        {
+            var response = new NullContentStreamMockResponse(200);
+            var transport = new MockTransport(_ => response);
+            var tableClient = new TableClient(_url, TableName, new MockCredential(), new TableClientOptions { Transport = transport });
+
+            RequestFailedException ex = Assert.ThrowsAsync<RequestFailedException>(async () => await tableClient.GetEntityIfExistsAsync<TableEntity>("pk", "rk-1"));
+            Assert.AreEqual(200, ex.Status);
+            StringAssert.Contains("200", ex.Message);
+        }
+
+        /// <summary>
+        /// Regression test for https://github.com/Azure/azure-sdk-for-net/issues/58303.
+        /// Verifies that GetEntityAsync throws RequestFailedException instead of
+        /// ArgumentNullException when the HTTP response has a null ContentStream, and that
+        /// the exception message includes the HTTP status code.
+        /// </summary>
+        [Test]
+        public async Task GetEntityAsyncThrowsRequestFailedExceptionWhenContentStreamIsNull()
+        {
+            var response = new NullContentStreamMockResponse(200);
+            var transport = new MockTransport(_ => response);
+            var tableClient = new TableClient(_url, TableName, new MockCredential(), new TableClientOptions { Transport = transport });
+
+            RequestFailedException ex = Assert.ThrowsAsync<RequestFailedException>(async () => await tableClient.GetEntityAsync<TableEntity>("pk", "rk-1"));
+            Assert.AreEqual(200, ex.Status);
+            StringAssert.Contains("200", ex.Message);
+        }
+
+        /// <summary>
+        /// Regression test for https://github.com/Azure/azure-sdk-for-net/issues/58303.
+        /// Verifies that GetEntityIfExistsAsync throws RequestFailedException instead of
+        /// JsonException when the HTTP response has an empty (non-null) ContentStream.
+        /// </summary>
+        [Test]
+        public async Task GetEntityIfExistsAsyncThrowsRequestFailedExceptionWhenContentStreamIsEmpty()
+        {
+            var response = new MockResponse(200);
+            response.ContentStream = new MemoryStream();
+            var transport = new MockTransport(_ => response);
+            var tableClient = new TableClient(_url, TableName, new MockCredential(), new TableClientOptions { Transport = transport });
+
+            RequestFailedException ex = Assert.ThrowsAsync<RequestFailedException>(async () => await tableClient.GetEntityIfExistsAsync<TableEntity>("pk", "rk-1"));
+            Assert.AreEqual(200, ex.Status);
+            StringAssert.Contains("200", ex.Message);
+        }
+
+        /// <summary>
+        /// Regression test for https://github.com/Azure/azure-sdk-for-net/issues/58303.
+        /// Verifies that GetEntityAsync throws RequestFailedException instead of
+        /// JsonException when the HTTP response has an empty (non-null) ContentStream.
+        /// </summary>
+        [Test]
+        public async Task GetEntityAsyncThrowsRequestFailedExceptionWhenContentStreamIsEmpty()
+        {
+            var response = new MockResponse(200);
+            response.ContentStream = new MemoryStream();
+            var transport = new MockTransport(_ => response);
+            var tableClient = new TableClient(_url, TableName, new MockCredential(), new TableClientOptions { Transport = transport });
+
+            RequestFailedException ex = Assert.ThrowsAsync<RequestFailedException>(async () => await tableClient.GetEntityAsync<TableEntity>("pk", "rk-1"));
+            Assert.AreEqual(200, ex.Status);
+            StringAssert.Contains("200", ex.Message);
+        }
+
         /// <summary>
         /// A mock response that disposes its ContentStream on Dispose(), simulating the
         /// behavior of the real HttpClientTransportResponse for non-MemoryStream seekable
@@ -801,6 +873,20 @@ public override void Dispose()
             }
         }
 
+        /// <summary>
+        /// A mock response with a null ContentStream. Used by regression tests for issue #58303.
+        /// </summary>
+        private class NullContentStreamMockResponse : MockResponse
+        {
+            public NullContentStreamMockResponse(int status) : base(status) { }
+
+            public override Stream ContentStream
+            {
+                get => null;
+                set { }
+            }
+        }
+
         public class EnumEntity : ITableEntity
         {
             public string PartitionKey { get; set; }