Add intialized notification.

[galachad]

This MR is necessary for standard operations.

Currently, the tools list request ends with an error.

{"jsonrpc":"2.0","id":2,"error":{"code":-32602,"message":"Invalid request parameters","data":""}}

After successful initialization, the client MUST send an initialized notification to indicate it is ready to begin normal operations.

The client SHOULD NOT send requests other than pings before the server has responded to the initialize request.

Reference: https://modelcontextprotocol.io/specification/2025-06-18/basic/lifecycle#lifecycle-phases

[simonpcouch]

Nice, thank you!

Somehow I remember both the client (and server reply) initialized notification as a MAY, but this doesn't seem to be the case in any of the versions on the linked webpage. Strange.

Will go ahead and approve GHA—I believe roundtrip may be broken with this PR and we'll need to implement a reply in the server.