fix: exclude null fields from streaming chunks for Open WebUI compatibility

OpenAI's API omits null fields in streaming chunks, but Tatlock was
including them (content: null, reasoning_content: null). This caused
parsing issues in Open WebUI's streaming handler.

Co-Authored-By: Claude Opus 4.5 <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()}\n\n"
+            yield f"data: {chunk.model_dump_json(exclude_none=True)}\n\n"
 
         yield "data: [DONE]\n\n"