refactor: 统一代理配置优先级链(key>provider>系统默认)并复用 HTTP 连接池

- 引入 resolve_proxy_param / build_proxy_client_kwargs 工具函数，统一
  httpx 客户端的代理+SSL+超时配置，替换各模块中零散的 get_ssl_context() 调用
- 所有涉及上游请求的模块（provider_query, usage replay, endpoint check,
  model fetch, OAuth, Vertex Auth, Gemini Files/Video 等）改用
  resolve_effective_proxy 按 key > provider > 系统默认优先级解析代理
- 流式请求改用 HTTPClientPool.get_upstream_client 复用连接池，移除各处
  http_client.aclose() 避免关闭共享客户端
- StreamProcessor._cleanup 不再关闭池中客户端，仅清理响应上下文
- 前端 EndpointFormDialog 增加 body_rules 帮助说明 Popover
- Mock handler 补充 OAuth 字段、endpoint extras 及新增 mock 路由

src/api/public/gemini_files.py

@@ -778,7 +778,11 @@ async def download_file(
 
     # 使用 follow_redirects=True 跟随重定向（Gemini 文件下载会重定向）
     try:
-        async with httpx.AsyncClient(follow_redirects=True, timeout=httpx.Timeout(300.0)) as client:
+        from src.services.proxy_node.resolver import build_proxy_client_kwargs
+
+        async with httpx.AsyncClient(
+            **build_proxy_client_kwargs(timeout=httpx.Timeout(300.0), follow_redirects=True)
+        ) as client:
             response = await client.get(upstream_url, headers=headers)
     except Exception as exc:
         logger.error("Gemini Files download failed: {}", exc)