test(inference): cover null-only message fields

Signed-off-by: Prekshi Vyas <prekshiv@nvidia.com>

Author: prekshivyas

src/lib/inference/health.test.ts

@@ -295,6 +295,11 @@ describe("inference health", () => {
       ],
       ["numeric streaming delta", 'data: {"choices":[{"delta":{"content":123}}]}\n'],
       ["a null content field and no tool call", '{"choices":[{"message":{"content":null}}]}'],
+      [
+        "a null reasoning field and no tool call",
+        '{"choices":[{"message":{"reasoning_content":null}}]}',
+      ],
+      ["a null refusal field and no tool call", '{"choices":[{"message":{"refusal":null}}]}'],
     ])("rejects a Chat Completions response with %s", (_description, body) => {
       const result = probeRemoteProviderHealth("openai-api", {
         model: "gpt-4o-mini",