refactor: 重构格式转换系统为 Hub-and-Spoke Normalizer 架构

- 移除旧的 converters 模块，改用基于 Internal 中间表示的 Normalizer 模式
- 新增 internal.py 定义统一的内部数据结构（InternalRequest/Response/Error）
- 新增 normalizer.py 定义 FormatNormalizer 基类接口
- 实现 OpenAI/Claude/Gemini 及其 CLI 格式的 Normalizer
- 新增 stream_events.py 和 stream_state.py 支持流式转换
- 重构 registry.py 使用 Normalizer 进行格式转换
- 更新 handlers 适配新的转换架构
- 前端移除 CLI 格式转换按钮的限制
- 添加 golden data 测试确保转换正确性

tests/services/test_format_conversion_candidate_selection.py

@@ -1,7 +1,8 @@
 import pytest
 from unittest.mock import AsyncMock, MagicMock
 
-from src.core.api_format import APIFormat, register_all_converters
+from src.core.api_format import APIFormat
+from src.core.api_format.conversion import register_default_normalizers
 from src.services.cache.aware_scheduler import CacheAwareScheduler
 
 
@@ -26,7 +27,7 @@ def _mock_endpoint(api_format: str, config: dict | None = None) -> MagicMock:
 
 @pytest.mark.asyncio
 async def test_build_candidates_blocks_cross_format_when_global_switch_off() -> None:
-    register_all_converters()
+    register_default_normalizers()
 
     scheduler = CacheAwareScheduler()
     scheduler._check_model_support = AsyncMock(return_value=(True, None, None, {"m"}))  # type: ignore[attr-defined]
@@ -56,7 +57,7 @@ async def test_build_candidates_blocks_cross_format_when_global_switch_off() ->
 
 @pytest.mark.asyncio
 async def test_build_candidates_includes_cross_format_when_enabled() -> None:
-    register_all_converters()
+    register_default_normalizers()
 
     scheduler = CacheAwareScheduler()
     scheduler._check_model_support = AsyncMock(return_value=(True, None, None, {"m"}))  # type: ignore[attr-defined]
@@ -88,7 +89,7 @@ async def test_build_candidates_includes_cross_format_when_enabled() -> None:
 
 @pytest.mark.asyncio
 async def test_exact_matches_rank_before_convertible() -> None:
-    register_all_converters()
+    register_default_normalizers()
 
     scheduler = CacheAwareScheduler()
     scheduler._check_model_support = AsyncMock(return_value=(True, None, None, {"m"}))  # type: ignore[attr-defined]