fix(vertex): SA 认证注入代理配置，细化 token 获取异常处理

- _auth_service_account 接收 endpoint 参数，通过 _get_proxy_config 解析代理
- vertex_auth 区分 TimeoutException/RequestError/通用异常，提供可读错误信息
- 新增测试覆盖代理传递和超时场景

tests/services/test_vertex_auth.py

@@ -0,0 +1,43 @@
+from __future__ import annotations
+
+import json
+
+import httpx
+import pytest
+
+from src.core.vertex_auth import VertexAuthError, VertexAuthService
+
+
+class _TimeoutAsyncClient:
+    def __init__(self, **_: object) -> None:
+        pass
+
+    async def __aenter__(self) -> "_TimeoutAsyncClient":
+        return self
+
+    async def __aexit__(self, exc_type: object, exc: object, tb: object) -> bool:
+        return False
+
+    async def post(self, *args: object, **kwargs: object) -> object:
+        raise httpx.ReadTimeout("")
+
+
+@pytest.mark.asyncio
+async def test_vertex_auth_timeout_error_includes_readable_message(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    service = VertexAuthService(
+        json.dumps(
+            {
+                "client_email": "svc@example.iam.gserviceaccount.com",
+                "private_key": "not-used-in-test",
+                "project_id": "demo-project",
+            }
+        )
+    )
+
+    monkeypatch.setattr(service, "_create_jwt", lambda: "signed-jwt")
+    monkeypatch.setattr("src.core.vertex_auth.httpx.AsyncClient", _TimeoutAsyncClient)
+
+    with pytest.raises(VertexAuthError, match=r"request timed out after 30s"):
+        await service.get_access_token(httpx_client_kwargs={"timeout": 30})