refactor: 推理协议层重构为策略/建造者模式

- ProtocolHandler 改为具体类，格式化委托给 ResponseBuilder
- 新增 api/protocols/ 目录，含 OpenAIResponseBuilder、AnthropicResponseBuilder
- GenContext、StopInfo 参数对象替代 StreamContext
- 消除 Builder 的实例可变状态（accumulated、_yielded）
- SSE 工具和停止检测收归 ProtocolHandler 统一管理
- prepare() 方法合并原来的 build_prompt、create_response_id
- 参数校验去重：仅 GenerationRequest.init 负责校验
- Prefill 阶段提前短路完全命中的缓存任务

astrai/inference/api/__init__.py

@@ -1,12 +1,6 @@
-"""Inference API: protocol handlers and FastAPI server."""
+"""Inference API: protocol handler, stop checker, and FastAPI server."""
 
-from astrai.inference.api.protocol import (
-    AnthropicHandler,
-    OpenAIHandler,
-    ProtocolHandler,
-    StopChecker,
-    StreamContext,
-)
+from astrai.inference.api.protocol import GenContext, ProtocolHandler, StopChecker
 from astrai.inference.api.server import (
     AnthropicMessage,
     ChatCompletionRequest,
@@ -17,11 +11,9 @@ from astrai.inference.api.server import (
 )
 
 __all__ = [
-    "AnthropicHandler",
-    "OpenAIHandler",
     "ProtocolHandler",
     "StopChecker",
-    "StreamContext",
+    "GenContext",
     "AnthropicMessage",
     "ChatCompletionRequest",
     "ChatMessage",