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/services/request/executor.py

@@ -175,14 +175,20 @@ class RequestExecutor:
                     )
                 else:
                     # 非流式请求：标记为 success 状态
-                    from src.services.proxy_node.resolver import resolve_proxy_info
+                    from src.services.proxy_node.resolver import (
+                        resolve_effective_proxy,
+                        resolve_proxy_info,
+                    )
 
+                    _eff_proxy = resolve_effective_proxy(
+                        getattr(provider, "proxy", None), getattr(key, "proxy", None)
+                    )
                     _extra: dict[str, Any] = {
                         "is_cached_user": is_cached_user,
                         "model_name": model_name,
                         "api_format": api_format,
                     }
-                    _pi = resolve_proxy_info(getattr(provider, "proxy", None))
+                    _pi = resolve_proxy_info(_eff_proxy)
                     if _pi:
                         _extra["proxy"] = _pi
                     RequestCandidateService.mark_candidate_success(