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/server.py

@@ -15,7 +15,9 @@ import uvicorn
 from fastapi import FastAPI, HTTPException
 from pydantic import BaseModel, Field
 
-from astrai.inference.api.protocol import AnthropicHandler, OpenAIHandler
+from astrai.inference.api.anthropic import AnthropicResponseBuilder
+from astrai.inference.api.openai import OpenAIResponseBuilder
+from astrai.inference.api.protocol import ProtocolHandler
 from astrai.inference.engine import InferenceEngine
 from astrai.model import AutoModel
 from astrai.tokenize import AutoTokenizer
@@ -133,14 +135,14 @@ async def get_stats():
 @app.post("/v1/chat/completions")
 async def chat_completion(request: ChatCompletionRequest):
     engine = _get_engine()
-    handler = OpenAIHandler(request, engine)
+    handler = ProtocolHandler(request, engine, OpenAIResponseBuilder())
     return await handler.handle()
 
 
 @app.post("/v1/messages")
 async def create_message(request: MessagesRequest):
     engine = _get_engine()
-    handler = AnthropicHandler(request, engine)
+    handler = ProtocolHandler(request, engine, AnthropicResponseBuilder())
     return await handler.handle()