refactor(proxy): Improve error handling and response writing in proxy handler

- Modify error response writing to use explicit WriteHeader and Write methods
- Remove redundant WriteHeader call in ServeHTTP method
- Add WriteHeader before writing response for both small and large responses
- Enhance code readability and error handling consistency

internal/handler/proxy.go

@@ -228,19 +228,19 @@ type ProxyHandler struct {
 	rateLimiter  *rateLimitManager
 }
 
-// 修改参数类型
+// NewProxyHandler 创建新的代理处理器
 func NewProxyHandler(cfg *config.Config) *ProxyHandler {
 	dialer := &net.Dialer{
-		Timeout:   clientConnTimeout, // 客户端连接超时
+		Timeout:   clientConnTimeout,
-		KeepAlive: 30 * time.Second,  // TCP keepalive 间隔
+		KeepAlive: 30 * time.Second,
 	}
 
 	transport := &http.Transport{
 		DialContext:           dialer.DialContext,
 		MaxIdleConns:          200,
 		MaxIdleConnsPerHost:   20,
-		IdleConnTimeout:       idleConnTimeout,     // 空闲连接超时
+		IdleConnTimeout:       idleConnTimeout,
-		TLSHandshakeTimeout:   tlsHandshakeTimeout, // TLS握手超时
+		TLSHandshakeTimeout:   tlsHandshakeTimeout,
 		ExpectContinueTimeout: 1 * time.Second,
 		MaxConnsPerHost:       50,
 		DisableKeepAlives:     false,
@@ -248,14 +248,14 @@ func NewProxyHandler(cfg *config.Config) *ProxyHandler {
 		ForceAttemptHTTP2:     true,
 		WriteBufferSize:       64 * 1024,
 		ReadBufferSize:        64 * 1024,
-		ResponseHeaderTimeout: backendServTimeout, // 后端服务响应头超时
+		ResponseHeaderTimeout: backendServTimeout,
 	}
 
 	return &ProxyHandler{
 		pathMap: cfg.MAP,
 		client: &http.Client{
 			Transport: transport,
-			Timeout:   proxyRespTimeout, // 整体代理响应超时
+			Timeout:   proxyRespTimeout,
 			CheckRedirect: func(req *http.Request, via []*http.Request) error {
 				if len(via) >= 10 {
 					return fmt.Errorf("stopped after 10 redirects")
@@ -271,9 +271,11 @@ func NewProxyHandler(cfg *config.Config) *ProxyHandler {
 		errorHandler: func(w http.ResponseWriter, r *http.Request, err error) {
 			log.Printf("[Error] %s %s -> %v", r.Method, r.URL.Path, err)
 			if strings.Contains(err.Error(), "rate limit exceeded") {
-				http.Error(w, "Too Many Requests", http.StatusTooManyRequests)
+				w.WriteHeader(http.StatusTooManyRequests)
+				w.Write([]byte("Too Many Requests"))
 			} else {
-				http.Error(w, "Internal Server Error", http.StatusInternalServerError)
+				w.WriteHeader(http.StatusInternalServerError)
+				w.Write([]byte("Internal Server Error"))
 			}
 		},
 	}
@@ -465,9 +467,6 @@ func (h *ProxyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	// 删除严格的 CSP
 	w.Header().Del("Content-Security-Policy")
 
-	// 设置响应状态码
-	w.WriteHeader(resp.StatusCode)
-
 	// 根据响应大小选择不同的处理策略
 	contentLength := resp.ContentLength
 	if contentLength > 0 && contentLength < 1<<20 { // 1MB 以下的小响应
@@ -482,7 +481,8 @@ func (h *ProxyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 
-		// 一次性写入响应
+		// 设置响应状态码并一次性写入响应
+		w.WriteHeader(resp.StatusCode)
 		written, err := w.Write(buf.Bytes())
 		if err != nil {
 			if !isConnectionClosed(err) {
@@ -492,6 +492,7 @@ func (h *ProxyHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		collector.RecordRequest(r.URL.Path, resp.StatusCode, time.Since(start), int64(written), utils.GetClientIP(r), r)
 	} else {
 		// 大响应使用零拷贝传输
+		w.WriteHeader(resp.StatusCode)
 		var bytesCopied int64
 		var err error