fix: send SSE heartbeat to prevent WebChat disconnect during compression

When context compression triggers with slow reasoning models (e.g.
deepseek-reasoner), the backend can go 30+ seconds without pushing any
SSE data. The client-side EventSource / browser then assumes the
connection is dead and disconnects, causing the WebUI to hang
indefinitely since it never receives the eventual response.

Fix: yield an SSE comment (`: heartbeat`) on every empty poll cycle.
Comment lines are defined in the SSE spec as keep-alive signals --
the EventSource API ignores them but the HTTP connection stays open.

Fixes #6938

Author: he-yufeng

astrbot/dashboard/routes/chat.py

@@ -40,6 +40,9 @@ async def _poll_webchat_stream_result(back_queue, username: str):
     try:
         result = await asyncio.wait_for(back_queue.get(), timeout=1)
     except asyncio.TimeoutError:
+        # Return a sentinel so the caller can send an SSE heartbeat to
+        # keep the connection alive during long-running operations (e.g.
+        # context compression with reasoning models).  See #6938.
         return None, False
     except asyncio.CancelledError:
         logger.debug(f"[WebChat] 用户 {username} 断开聊天长连接。")
@@ -364,6 +367,11 @@ async def stream():
                             client_disconnected = True
                             break
                         if not result:
+                            # Send an SSE comment as keep-alive so the client
+                            # doesn't time out during slow backend ops like
+                            # context compression with reasoning models (#6938).
+                            if not client_disconnected:
+                                yield ": heartbeat\n\n"
                             continue
 
                         if (