fix: use exclude_unset for SSE streaming chunk serialization

exclude_none was too aggressive — it stripped finish_reason: null from
intermediate chunks (which OpenAI includes). exclude_unset correctly
omits only fields never passed to the constructor (like reasoning_content
on content-only chunks) while preserving explicitly-set finish_reason: null.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/chat/router.py

@@ -31,7 +31,7 @@ async def _stream_response(
     """
     try:
         async for chunk in service.create_chat_completion_stream(request):
-            yield f"data: {chunk.model_dump_json(exclude_none=True)}\n\n"
+            yield f"data: {chunk.model_dump_json(exclude_unset=True)}\n\n"
 
         yield "data: [DONE]\n\n"