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/factory.py

@@ -3,12 +3,14 @@
 根据运行平台自动选择最优传输实现。
 """
 
+from typing import Optional
+
 import sys
 
 from .base import TransportClient, TransportServer
 
 
-def create_transport_server(socket_path: str | None = None) -> TransportServer:
+def create_transport_server(socket_path: Optional[str] = None) -> TransportServer:
     """创建传输服务端
 
     Linux/macOS 使用 UDS，Windows 使用 TCP 回退。