feat(chat): stream model thinking over SSE and show it in a collapsible block

tests/unit/test_chat_gate.py

@@ -20,6 +20,7 @@ from app.api import chat as chat_api
 from app.config import Settings
 from app.main import app as fastapi_app
 from app.models import Document, QueryLog
+from app.rag.llm import StreamPiece
 from app.rag.retriever import RetrievedChunk, weak_hit_titles
 from app.rag.suggestions import MAX_SUGGESTIONS, derive_suggestions
 
@@ -276,7 +277,7 @@ class _CannedLLM:
     async def chat_stream(self, messages: list[dict[str, str]]):
         self.seen.append(messages)
         for i in range(0, len(self.answer), 12):
-            yield self.answer[i : i + 12]
+            yield StreamPiece("content", self.answer[i : i + 12])
 
 
 class _FakeSteeringResult: