refactor: use background task for client disconnection monitoring

- Replace time-based throttling with background task for disconnect checks
- Remove time.monotonic() and related throttling logic
- Prevent blocking of stream transmission during connection checks
- Properly clean up background task with try/finally block
- Improve throughput and responsiveness of stream processing

src/api/handlers/base/stream_processor.py

@@ -11,7 +11,6 @@
 import asyncio
 import codecs
 import json
-import time
 from typing import Any, AsyncGenerator, Callable, Optional
 
 import httpx
@@ -407,27 +406,40 @@ class StreamProcessor:
             响应数据块
         """
         try:
-            # 断连检查频率：每次 await 都会引入调度开销，过于频繁会让流式"发一段停一段"
-            # 这里按时间间隔节流，兼顾及时停止上游读取与吞吐平滑性。
-            next_disconnect_check_at = 0.0
-            disconnect_check_interval_s = 0.25
+            # 使用后台任务检查断连，完全不阻塞流式传输
+            disconnected = False
 
-            async for chunk in stream_generator:
-                now = time.monotonic()
-                if now >= next_disconnect_check_at:
-                    next_disconnect_check_at = now + disconnect_check_interval_s
+            async def check_disconnect_background() -> None:
+                nonlocal disconnected
+                while not disconnected and not ctx.has_completion:
+                    await asyncio.sleep(0.5)
                     if await is_disconnected():
-                        # 如果响应已完成（收到 finish_reason），客户端断开不算失败
+                        disconnected = True
+                        break
+
+            # 启动后台检查任务
+            check_task = asyncio.create_task(check_disconnect_background())
+
+            try:
+                async for chunk in stream_generator:
+                    if disconnected:
+                        # 如果响应已完成，客户端断开不算失败
                         if ctx.has_completion:
                             logger.info(
                                 f"ID:{self.request_id} | Client disconnected after completion"
                             )
                         else:
                             logger.warning(f"ID:{self.request_id} | Client disconnected")
-                            ctx.status_code = 499  # Client Closed Request
+                            ctx.status_code = 499
                             ctx.error_message = "client_disconnected"
                         break
-                yield chunk
+                    yield chunk
+            finally:
+                check_task.cancel()
+                try:
+                    await check_task
+                except asyncio.CancelledError:
+                    pass
         except asyncio.CancelledError:
             # 如果响应已完成，不标记为失败
             if not ctx.has_completion: