feat(engine): #873 dual-Claw event contract + SseAdapter/BufferAdapter + chat.py SSE delegate + im_llm EngineCore thin-shell (gray-gated)

- app/core/engine_event.py (#880): EngineEvent dataclass + make_* factories, single construction entry, byte-compatible wire with legacy ChatPage.tsx
- app/core/claw_adapters.py (#884): SseAdapter (hot frames zero-metadata, terminal keeps trace_id) + BufferAdapter (assemble() preserves __media__)
- app/api/chat.py (#881): /messages/stream now delegates to SseAdapter; wire bytes unchanged (46B delta / done+message_id)
- app/api/im_llm.py (#883): IM_USE_ENGINE_CORE gray gate; _call_engine_core reuses EngineCore via BufferAdapter (default OFF, preserves media fast-path)
- app/api/chat_service.py: capture 3 container-ahead fixes (stray import asyncio removed, logger.info format, LiteLLM master key)

backend/python/app/api/chat.py

@@ -84,6 +84,7 @@ from ..auth import get_current_user
 from ..platform_knowledge import get_platform_knowledge
 
 from ..mcp_client import list_mcp_tools, call_mcp_tool
+from ..core.claw_adapters import SseAdapter
 
 from ..middleware.log_service import LogService
 
@@ -1570,38 +1571,37 @@ async def send_message_stream(
     req: SendMessageRequest,
     current_user: User = Depends(get_current_user),
 ):
-    """SSE 流式聊天端点(Phase 0)。
-    与普通 /messages 共用 _send_message_core，仅通过 stream=True + on_event
-    回调实时吐出增量(delta/reasoning)，末尾推 done 事件(含 message_id)。
+    """SSE 流式聊天端点(#873)。
+
+    与 /messages 共用 EngineCore(_send_message_core)，仅通过 stream=True +
+    on_event=SseAdapter 实时吐出增量(delta/reasoning/step)，末尾推 done 终帧
+    (含 message_id 等)。SseAdapter 保证线格式与改造前逐字节一致。
     """
     db = SessionLocal()
     try:
-        queue = asyncio.Queue()
-
-        async def on_event(e):
-            await queue.put(e)
+        adapter = SseAdapter(
+            trace_id=f"sse-{conv_id}",
+            tenant_id=str(getattr(current_user, "tenant_id", "") or ""),
+        )
 
         async def runner():
             try:
                 await _send_message_core(
                     conv_id, req, db, current_user,
-                    stream=True, on_event=on_event,
+                    stream=True, on_event=adapter,
                 )
             except Exception as exc:
                 logger.warning(f"[sse] chat stream error: {exc}", exc_info=True)
-                await queue.put({"type": "error", "message": str(exc)})
+                await adapter.emit_error(str(exc))
             finally:
-                await queue.put({"type": "__end__"})
+                await adapter.close()
 
         task = asyncio.create_task(runner())
 
         async def event_gen():
             try:
-                while True:
-                    e = await queue.get()
-                    if e.get("type") == "__end__":
-                        break
-                    yield f"data: {json.dumps(e, ensure_ascii=False)}\n\n"
+                async for frame in adapter.frames():
+                    yield frame
             finally:
                 await task
                 db.close()
@@ -1611,10 +1611,6 @@ async def send_message_stream(
         db.close()
         raise
 
-
-
-
-
 @router.post("/conversations/{conv_id}/clear")