feat: emit real status events from on_chain_start (thinking/generating)

- Add has_tool_activity flag to distinguish first agent pass (thinking)
  from post-tool agent pass (generating)
- Replace on_chain_start debug logging with status SSE emission
- Filter to only graph-level agent nodes via graph:step: tag prefix

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/chat.py

@@ -81,6 +81,7 @@ async def _stream_response(request: ChatRequest) -> AsyncIterator[bytes]:
     full_content: list[str] = []
     logger = logging.getLogger(__name__)
     tool_start_ts: dict[str, int] = {}   # track per-tool start timestamps
+    has_tool_activity = False
 
     try:
         async for event in graph.astream_events(
@@ -104,6 +105,7 @@ async def _stream_response(request: ChatRequest) -> AsyncIterator[bytes]:
                 tool_input = event.get("data", {}).get("input", {})
                 ts = int(time.time() * 1000)
                 tool_start_ts[call_id] = ts
+                has_tool_activity = True
                 yield _sse({
                     "type": "tool_start",
                     "call_id": call_id,
@@ -144,14 +146,21 @@ async def _stream_response(request: ChatRequest) -> AsyncIterator[bytes]:
 
             elif kind == "on_chain_start":
                 chain_name = event.get("name", "")
-                metadata = event.get("metadata", {})
                 tags = event.get("tags", [])
-                logger.warning(
-                    "on_chain_start: name=%s, langgraph_node=%s, tags=%s",
-                    chain_name,
-                    metadata.get("langgraph_node", ""),
-                    tags,
-                )
+                is_graph_step = any(t.startswith("graph:step:") for t in tags)
+                if chain_name == "agent" and is_graph_step:
+                    if has_tool_activity:
+                        yield _sse({
+                            "type": "status",
+                            "stage": "generating",
+                            "message": "正在生成回复...",
+                        })
+                    else:
+                        yield _sse({
+                            "type": "status",
+                            "stage": "thinking",
+                            "message": "正在分析...",
+                        })
 
     except Exception as exc:
         logger.error("SSE stream error for conversation %s: %s", request.conversation_id, exc, exc_info=True)