refactor: 统一请求头处理逻辑到 headers.py

- 新增 src/core/headers.py 集中管理头部处理函数
- 扩展 api_format_metadata.py 添加 extra_headers 和 protected_keys 配置
- 将各 adapter 中重复的头部方法提取到基类，统一调用 headers.py
- 移除 transport.py 中未使用的 build_provider_headers 函数
- 添加 headers 模块的单元测试

src/api/handlers/gemini_cli/adapter.py

@@ -4,10 +4,9 @@ Gemini CLI Adapter - 基于通用 CLI Adapter 基类的实现
 继承 CliAdapterBase，处理 Gemini CLI 格式的请求。
 """
 
-from typing import Any, AsyncIterator, Dict, Optional, Tuple, Type, Union
+from typing import Any, Dict, Optional, Tuple, Type
 
 import httpx
-from fastapi import Request
 
 from src.api.handlers.base.cli_adapter_base import CliAdapterBase, register_cli_adapter
 from src.api.handlers.base.cli_handler_base import CliMessageHandlerBase
@@ -37,10 +36,6 @@ class GeminiCliAdapter(CliAdapterBase):
     def __init__(self, allowed_api_formats: Optional[list[str]] = None):
         super().__init__(allowed_api_formats or ["GEMINI_CLI"])
 
-    def extract_api_key(self, request: Request) -> Optional[str]:
-        """从请求中提取 API 密钥 (x-goog-api-key)"""
-        return request.headers.get("x-goog-api-key")
-
     def _merge_path_params(
         self, original_request_body: Dict[str, Any], path_params: Dict[str, Any]  # noqa: ARG002
     ) -> Dict[str, Any]:
@@ -138,19 +133,6 @@ class GeminiCliAdapter(CliAdapterBase):
             prefix = f"{base_url}/v1beta"
         return f"{prefix}/models/{effective_model_name}:generateContent"
 
-    @classmethod
-    def build_base_headers(cls, api_key: str) -> Dict[str, str]:
-        """构建Gemini CLI API认证头"""
-        return {
-            "x-goog-api-key": api_key,
-            "Content-Type": "application/json",
-        }
-
-    @classmethod
-    def get_protected_header_keys(cls) -> tuple:
-        """返回Gemini CLI API的保护头部key"""
-        return ("x-goog-api-key", "content-type")
-
     @classmethod
     def build_request_body(cls, request_data: Dict[str, Any]) -> Dict[str, Any]:
         """构建Gemini CLI API请求体"""