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/services/orchestration/candidate_resolver.py

@@ -10,7 +10,7 @@ from typing import Any, Dict, List, Optional, Tuple
 
 from sqlalchemy.orm import Session
 
-from src.core.enums import APIFormat
+from src.core.api_format import APIFormat
 from src.core.exceptions import ProviderNotAvailableException
 from src.core.logger import logger
 from src.models.database import ApiKey

src/services/orchestration/error_classifier.py

@@ -11,7 +11,7 @@ from typing import Any, Dict, Optional, Tuple, Union
 import httpx
 from sqlalchemy.orm import Session
 
-from src.core.enums import APIFormat
+from src.core.api_format import APIFormat
 from src.core.exceptions import (
     ConcurrencyLimitError,
     ProviderAuthException,

src/services/orchestration/fallback_orchestrator.py

@@ -29,7 +29,7 @@ import httpx
 from redis import Redis
 from sqlalchemy.orm import Session
 
-from src.core.enums import APIFormat
+from src.core.api_format import APIFormat
 from src.core.error_utils import extract_error_message
 from src.core.exceptions import (
     ConcurrencyLimitError,

src/services/orchestration/request_dispatcher.py

@@ -8,7 +8,7 @@ from typing import Any, Callable, Optional, Tuple
 
 from sqlalchemy.orm import Session
 
-from src.core.enums import APIFormat
+from src.core.api_format import APIFormat
 from src.core.logger import logger
 from src.models.database import ApiKey
 from src.services.cache.aware_scheduler import CacheAwareScheduler, ProviderCandidate