refactor: 重构 API 格式相关代码到统一的 api_format 模块

- 新增 src/core/api_format/ 模块，整合所有 API 格式相关功能
  - enums.py: APIFormat 枚举定义
  - metadata.py: API 格式元数据注册
  - headers.py: 请求头处理逻辑
  - detection.py: API 格式检测
  - conversion/: 格式转换器（Claude/OpenAI/Gemini 互转）
- 删除分散在各处的旧实现
  - src/core/api_format_metadata.py
  - src/core/headers.py
  - src/api/handlers/*/converter.py
  - src/api/handlers/base/format_converter_registry.py
- 更新所有引用路径，使用新的统一模块
- 新增 cli_handler_base 中的格式转换支持

src/models/admin_requests.py

@@ -10,7 +10,8 @@ from typing import Any, Dict, List, Optional
 
 from pydantic import BaseModel, Field, field_validator, model_validator
 
-from src.core.enums import APIFormat, ProviderBillingType
+from src.core.api_format import APIFormat
+from src.core.enums import ProviderBillingType
 
 
 class ProxyConfig(BaseModel):

src/models/endpoint_models.py

@@ -49,7 +49,7 @@ class ProviderEndpointCreate(BaseModel):
     @classmethod
     def validate_api_format(cls, v: str) -> str:
         """验证 API 格式"""
-        from src.core.enums import APIFormat
+        from src.core.api_format import APIFormat
 
         allowed = [fmt.value for fmt in APIFormat]
         v_upper = v.upper()
@@ -200,7 +200,7 @@ class EndpointAPIKeyCreate(BaseModel):
         if v is None:
             return v
 
-        from src.core.enums import APIFormat
+        from src.core.api_format import APIFormat
 
         allowed = [fmt.value for fmt in APIFormat]
         validated = []
@@ -335,7 +335,7 @@ class EndpointAPIKeyUpdate(BaseModel):
         if v is None:
             return v
 
-        from src.core.enums import APIFormat
+        from src.core.api_format import APIFormat
 
         allowed = [fmt.value for fmt in APIFormat]
         validated = []