Handle list_changed notifications instead of dropping them #2107

Summary

The SDK silently drops notifications/tools/list_changed, notifications/prompts/list_changed, and notifications/resources/list_changed notifications. They fall through to case _: pass in the notification handler.

At minimum, these should be surfaced — either via a callback, an event, or a hook — so that consumers can react to them (e.g., invalidate caches, refresh tool lists).

This is related to #1429 (dynamic tool management) and #710 (triggering listChanged), but distinct: those are about the server side sending notifications, this is about the client side receiving them.