Log warning on MCP protocol version mismatch

[jserv]

Summary

The server does not inspect the client's protocolVersion during initialize. Per MCP spec, the server returns its version and the client decides whether to disconnect — but a mismatch should be logged for debugging.

Details

• Fix: log a warning to stderr when the client's protocol version differs from the server's. No handshake behavior change.
• Gate: E2E test verifies initialize succeeds with a mismatched version.