perf: optimize JSON deserialization from stream

Replaced `StreamReader.ReadToEnd()` and string-based deserialization with
direct stream deserialization in `SystemTextJsonSerializer.Deserialize<T>(Stream)`.

This change avoids an intermediate string allocation for the entire JSON payload,
reducing memory pressure and improving performance, especially for larger responses.
The stream disposal logic is maintained using the `using (stream)` pattern.

Co-authored-by: antarr <974295+antarr@users.noreply.github.com>

Author: google-labs-jules[bot]

Withings.NET/Client/Authenticator.cs

@@ -124,10 +124,9 @@ public T Deserialize<T>(string s)
 
         public T Deserialize<T>(Stream stream)
         {
-             using (var reader = new StreamReader(stream))
+             using (stream)
              {
-                 var text = reader.ReadToEnd();
-                 return JsonSerializer.Deserialize<T>(text, _options);
+                 return JsonSerializer.Deserialize<T>(stream, _options);
              }
         }