fix(stream): 修复流式请求超时和取消归因问题

- 流式请求 timeout 改为 None，避免 provider.request_timeout 作为整条流总时长超时导致长响应被硬切断
- 重构 CancelledError 断连归因逻辑：提取探测方法并支持多次重试确认，降低误判率
- 新增 cancelled_unknown 状态处理断连检测不确定的场景，避免错误归因为 server_cancelled
- 在 upstream_response 中记录取消详情，便于排查
- 新增断连归因逻辑的单元测试

tests/api/handlers/base/test_cli_monitor_mixin.py

@@ -0,0 +1,87 @@
+from __future__ import annotations
+
+import asyncio
+from collections.abc import AsyncGenerator
+
+import pytest
+
+from src.api.handlers.base.cli_monitor_mixin import CliMonitorMixin
+from src.api.handlers.base.stream_context import StreamContext
+
+
+class _DummyMonitor(CliMonitorMixin):
+    pass
+
+
+class _RequestStub:
+    def __init__(self, responses: list[bool | Exception]):
+        self._responses = responses
+
+    async def is_disconnected(self) -> bool:
+        if self._responses:
+            value = self._responses.pop(0)
+        else:
+            value = False
+        if isinstance(value, Exception):
+            raise value
+        return value
+
+
+async def _cancel_immediately() -> AsyncGenerator[bytes, None]:
+    if False:
+        yield b""
+    raise asyncio.CancelledError()
+
+
+@pytest.mark.asyncio
+async def test_create_monitored_stream_marks_client_disconnected_when_confirmed() -> None:
+    monitor = _DummyMonitor()
+    monitor.CANCEL_DISCONNECT_RETRY_DELAYS_SECONDS = ()
+    ctx = StreamContext(model="test-model", api_format="openai:cli", request_id="req-client")
+
+    request = _RequestStub([True])
+    monitored = monitor._create_monitored_stream(ctx, _cancel_immediately(), request)
+
+    with pytest.raises(asyncio.CancelledError):
+        async for _ in monitored:
+            pass
+
+    assert ctx.status_code == 499
+    assert ctx.error_message == "client_disconnected"
+    assert "cancel_origin=client_disconnected" in (ctx.upstream_response or "")
+
+
+@pytest.mark.asyncio
+async def test_create_monitored_stream_marks_server_cancelled_when_confirmed_connected() -> None:
+    monitor = _DummyMonitor()
+    monitor.CANCEL_DISCONNECT_RETRY_DELAYS_SECONDS = ()
+    ctx = StreamContext(model="test-model", api_format="openai:cli", request_id="req-server")
+
+    request = _RequestStub([False])
+    monitored = monitor._create_monitored_stream(ctx, _cancel_immediately(), request)
+
+    with pytest.raises(asyncio.CancelledError):
+        async for _ in monitored:
+            pass
+
+    assert ctx.status_code == 503
+    assert ctx.error_message == "server_cancelled"
+    assert "cancel_origin=server_cancelled" in (ctx.upstream_response or "")
+
+
+@pytest.mark.asyncio
+async def test_create_monitored_stream_marks_cancelled_unknown_when_disconnect_check_uncertain() -> None:
+    monitor = _DummyMonitor()
+    monitor.CANCEL_DISCONNECT_RETRY_DELAYS_SECONDS = ()
+    ctx = StreamContext(model="test-model", api_format="openai:cli", request_id="req-unknown")
+
+    request = _RequestStub([asyncio.TimeoutError()])
+    monitored = monitor._create_monitored_stream(ctx, _cancel_immediately(), request)
+
+    with pytest.raises(asyncio.CancelledError):
+        async for _ in monitored:
+            pass
+
+    assert ctx.status_code == 503
+    assert ctx.error_message == "cancelled_unknown"
+    assert "cancel_origin=cancelled_unknown" in (ctx.upstream_response or "")