refactor: 拆分大型模块为 mixin/子模块结构

- cli_handler_base.py 拆分为 7 个 mixin (event/monitor/prefetch/request/sse_helpers/stream/sync)
- usage/service.py 拆分为 6 个子模块 (types/active_requests/cache_analysis/lifecycle/pricing/query/recording)
- models/database 拆分为独立模型文件 (auth/misc/model/provider/stats/usage/user)
- DUMMY_THOUGHT_SIGNATURE 常量提升到 core/api_format/conversion/constants 统一管理
- task/service.py 内联导入提升为顶层导入
- 流处理函数签名移除冗余的 http_client 参数

tests/api/handlers/base/test_stream_processor.py

@@ -84,12 +84,7 @@ def test_process_line_handles_openai_usage_chunk_followed_by_done_without_blank_
 
 
 class _DummyResponseCtx:
-    async def __aexit__(self, exc_type, exc, tb) -> None:  # noqa: ANN001
-        return None
-
-
-class _DummyHTTPClient:
-    async def aclose(self) -> None:
+    async def __aexit__(self, exc_type: type | None, exc: BaseException | None, tb: object) -> None:
         return None
 
 
@@ -115,7 +110,6 @@ async def test_create_response_stream_flushes_usage_on_remote_protocol_error() -
         ctx=ctx,
         byte_iterator=_iter_bytes_then_remote_protocol_error(),
         response_ctx=_DummyResponseCtx(),
-        http_client=_DummyHTTPClient(),  # type: ignore[arg-type]
         prefetched_chunks=[],
         start_time=None,
     ):