fix(inference): reject a reply whose only message field is null

`hasValidChatMessageFields` set `recognizedField` on the mere presence of a
`content`, `reasoning_content`, or `refusal` key, before the value was read.
`isValidChatContent` admits `null` because that is the shape OpenAI uses for a
pure tool call, so `{"choices":[{"message":{"role":"assistant","content":null}}]}`
satisfied the validator with no tool call anywhere in the message. `doctor` and
`status` reported "model-invocation probe succeeded" for a reply that carried
nothing, which is the shape a quota-exhausted or truncated hosted turn returns.

The same function already rejects the message when the key is absent, and its
own failure text states the rule it means to enforce: "no choice carried a
message with text content, a refusal, or tool calls". A nulled key carries
exactly as much proof as a missing one.

The flag now moves behind the value check, matching the rule the `tool_calls`
branch adopted in NVIDIA#9124: presence claims nothing, only a value does. A reply
that pairs `"content": null` with a real tool call still validates, because the
`tool_calls` branch sets the flag.

Signed-off-by: Udaya Tejas <udayatejas2004@gmail.com>

Author: udsy19

src/lib/inference/health.test.ts

@@ -294,6 +294,7 @@ describe("inference health", () => {
         '{"choices":[{"message":{"content":"OK","tool_calls":"none"}}]}',
       ],
       ["numeric streaming delta", 'data: {"choices":[{"delta":{"content":123}}]}\n'],
+      ["a null content field and no tool call", '{"choices":[{"message":{"content":null}}]}'],
     ])("rejects a Chat Completions response with %s", (_description, body) => {
       const result = probeRemoteProviderHealth("openai-api", {
         model: "gpt-4o-mini",

src/lib/inference/health.ts

@@ -218,11 +218,11 @@ function hasValidChatMessageFields(
   let recognizedField = false;
   for (const field of ["content", "reasoning_content", "refusal"] as const) {
     if (!(field in message)) continue;
-    recognizedField = true;
     const value = message[field];
     const valid =
       field === "content" ? isValidChatContent(value) : value === null || typeof value === "string";
     if (!valid) return false;
+    if (value !== null) recognizedField = true;
   }
   if ("tool_calls" in message) {
     const toolCalls = message.tool_calls;