feat(proxy,failover,transport): Hyper 上游客户端精细计时、连续失败退避与连接泄漏修复

Proxy:
- 将上游 HTTP 客户端从 reqwest 替换为 hyper，新增 InstrumentedConnector
  实现 TCP 连接/TLS 握手级别的独立计时，上报 connection_reused 等指标
- 前端展示细粒度代理计时（连接复用、等待响应头等）

Failover:
- 引入连续失败退避机制，每 10 次失败递增退避间隔
- 检测 H2 max outbound streams 错误并触发上游客户端重建
- 新增 HTTPClientPool.reset_upstream_client 支持按需重建缓存客户端

连接泄漏修复:
- Handler 异常路径确保 response_ctx 被正确关闭
- HubResponseStream 迭代结束后在 finally 块中清理 stream_id
- HubTunnelTransport.handle_request 捕获所有异常并清理流状态

src/api/handlers/base/chat_handler_base.py

@@ -1287,6 +1287,14 @@ class ChatHandlerBase(BaseMessageHandler, ABC):
                     response_ctx = None
                     continue
 
+                try:
+                    if response_ctx is not None:
+                        await response_ctx.__aexit__(None, None, None)
+                except Exception:
+                    pass
+                finally:
+                    response_ctx = None
+
                 from src.api.handlers.base.chat_sync_executor import ChatSyncExecutor
 
                 error_text = await ChatSyncExecutor(self)._extract_error_text(e)
@@ -1306,6 +1314,13 @@ class ChatHandlerBase(BaseMessageHandler, ABC):
                 raise
 
             except Exception:
+                try:
+                    if response_ctx is not None:
+                        await response_ctx.__aexit__(None, None, None)
+                except Exception:
+                    pass
+                finally:
+                    response_ctx = None
                 raise
 
         # 类型断言：成功执行后这些变量不会为 None

src/api/handlers/base/cli_stream_mixin.py

@@ -800,6 +800,14 @@ class CliStreamMixin:
                     response_ctx = None
                     continue
 
+                try:
+                    if response_ctx is not None:
+                        await response_ctx.__aexit__(None, None, None)
+                except Exception:
+                    pass
+                finally:
+                    response_ctx = None
+
                 error_text = await self._extract_error_text(e)
                 logger.error(
                     f"Provider 返回错误状态: {e.response.status_code}\n  Response: {error_text}"
@@ -818,6 +826,13 @@ class CliStreamMixin:
                 raise
 
             except Exception:
+                try:
+                    if response_ctx is not None:
+                        await response_ctx.__aexit__(None, None, None)
+                except Exception:
+                    pass
+                finally:
+                    response_ctx = None
                 raise
 
         # 类型断言：成功执行后这些变量不会为 None