fix: 修复 Chat Completions 编译错误和运行时 panic

1. 修复 WriteFilteredHeaders API 不兼容（2处）:
   将 s.cfg.Security.ResponseHeaders 改为 s.responseHeaderFilter，
   因为 main 分支已将函数签名改为接受 *responseheaders.CompiledHeaderFilter

2. 修复 writer 生命周期导致的 nil pointer panic:
   ChatCompletions handler 替换了 c.Writer 但未恢复，导致
   OpsErrorLogger 中间件的 defer 释放 opsCaptureWriter 后，
   Logger 中间件调用 c.Writer.Status() 触发空指针解引用。
   通过保存并恢复 originalWriter 修复。

3. 为 chatCompletionsResponseWriter 添加防御性 Status() 和
   Written() 方法，包含 nil 安全检查

4. 恢复 gateway.go 中被误删的 net/http import

backend/internal/handler/openai_chat_completions.go

@@ -62,6 +62,7 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 
 	stream, _ := converted["stream"].(bool)
 	model, _ := converted["model"].(string)
+	originalWriter := c.Writer
 	writer := newChatCompletionsResponseWriter(c.Writer, stream, includeUsage, model)
 	c.Writer = writer
 	c.Request.Body = io.NopCloser(bytes.NewReader(convertedBody))
@@ -69,6 +70,7 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 
 	h.Responses(c)
 	writer.Finalize()
+	c.Writer = originalWriter
 }
 
 type chatCompletionsResponseWriter struct {
@@ -167,6 +169,20 @@ func (w *chatCompletionsResponseWriter) SetPassthrough() {
 	w.passthrough = true
 }
 
+func (w *chatCompletionsResponseWriter) Status() int {
+	if w.ResponseWriter == nil {
+		return 0
+	}
+	return w.ResponseWriter.Status()
+}
+
+func (w *chatCompletionsResponseWriter) Written() bool {
+	if w.ResponseWriter == nil {
+		return false
+	}
+	return w.ResponseWriter.Written()
+}
+
 func (w *chatCompletionsResponseWriter) flushStreamBuffer() {
 	for {
 		buf := w.streamBuf.Bytes()