Add protocol_version field to Initialize RPC for forward compatibility as the plugin protocol evolves.
Changes:
- Add protocol_version to InitializeRequest (server → plugin)
- Add protocol_version to InitializeResponse (plugin → server)
- Set current protocol version to 1
- Version 0 indicates pre-versioning implementations (treated as v1)
This allows the server and plugins to negotiate capabilities:
- Server can avoid calling RPCs that older plugins don't implement
- Plugins can detect newer servers and enable advanced features
- Graceful degradation when versions mismatch
The AI service now reports protocol version 1 during initialization.