Handle shutdown and exit lifecycle methods per MCP spec

[jserv]

Summary

shutdown and exit are not matched in the dispatch loop and fall through to METHOD_NOT_FOUND (-32601), violating the MCP spec.

Details

• shutdown (request with id): MCP spec requires the server to respond {} then stop accepting new requests. Currently returns -32601.
• exit (notification): MCP spec requires unconditional process exit. Currently silently ignored.
• notifications/cancelled is already handled (silently dropped, correct per JSON-RPC). The in-progress scan is not aborted, which is acceptable for sync transport where scans are <500ms.
• Fix: add "shutdown" arm returning {} then break the serve loop; add "exit" arm calling process::exit(0). Apply to both sync and async transports.
• Gate: E2E test verifies shutdown → {} + clean exit; exit → process terminates.