refactor: 重构底层代码，抽离conn，调整消息处理器并创建传输层接口。

feature: 支持mqtt非桥接版本。

main/xiaozhi-server/core/pipeline/message_pipeline.py

@@ -0,0 +1,28 @@
+from abc import ABC, abstractmethod
+from typing import Any, List
+
+
+class MessageProcessor(ABC):
+    """消息处理器接口。返回 True 表示已处理并中止后续处理。"""
+
+    @abstractmethod
+    async def process(self, context: Any, transport: Any, message: Any) -> bool:
+        raise NotImplementedError
+
+
+class MessagePipeline:
+    """责任链式消息处理管道。"""
+
+    def __init__(self) -> None:
+        self._processors: List[MessageProcessor] = []
+
+    def add_processor(self, processor: MessageProcessor) -> None:
+        self._processors.append(processor)
+
+    async def process_message(self, context: Any, transport: Any, message: Any) -> None:
+        for processor in self._processors:
+            handled = await processor.process(context, transport, message)
+            if handled:
+                return
+
+