Refactor protocol and transport modules to use type hints for improved clarity and consistency

- Updated Codec class to use abstract methods for encoding and decoding envelopes.
- Changed Envelope class to use Dict and Optional for payload and error fields.
- Refined error handling in RPCError class with Optional type hints for details.
- Enhanced manifest validation logic with type hints for better type safety.
- Improved plugin loading mechanism with consistent type annotations.
- Updated RPCClient to utilize Optional for codec and connection attributes.
- Refactored transport classes to use Optional for server attributes and socket paths.

src/plugin_runtime/transport/uds.py

@@ -4,6 +4,7 @@
 """
 
 from pathlib import Path
+from typing import Optional
 
 import asyncio
 import os
@@ -20,13 +21,13 @@ class UDSConnection(Connection):
 class UDSTransportServer(TransportServer):
     """UDS 传输服务端"""
 
-    def __init__(self, socket_path: str | None = None):
+    def __init__(self, socket_path: Optional[str] = None):
         if socket_path is None:
             # 默认放在临时目录，使用 uuid 确保同一进程多实例不碰撞
             import uuid
             socket_path = os.path.join(tempfile.gettempdir(), f"maibot-plugin-{os.getpid()}-{uuid.uuid4().hex[:8]}.sock")
         self._socket_path = socket_path
-        self._server: asyncio.AbstractServer | None = None
+        self._server: Optional[asyncio.AbstractServer] = None
 
     async def start(self, handler: ConnectionHandler) -> None:
         # 清理残留 socket 文件