Tool descriptor updates, toolchange event, and registration error handling

[kashishhora]

Summary

Builds on the document.modelContext migration (#23). Updates the tool descriptor to the current spec shape, adds the toolchange event, and makes registration error handling robust:

• Annotations narrowed to { readOnlyHint?, untrustedContentHint? } and a top-level title added to the tool config/descriptor (the classic destructiveHint / idempotentHint / openWorldHint and annotations.title are dropped, matching the spec).
• exposedTo?: string[] is forwarded from the tool config to registerTool, and the tool re-registers when it changes.
• toolchange event on document.modelContext (now an EventTarget) — supports addEventListener('toolchange', …) and an ontoolchange handler. Notifications fan out to the testing shim and the event dispatcher.
• Registration error handling: rejections route into the hook's error state + onError (AbortError from teardown is ignored). Ownership uses compare-and-set so a duplicate-name registration can't evict the existing owner. schemaFingerprint is hardened against non-serializable schemas so a bad schema can't throw during render.

Test plan

• pnpm typecheck — clean
• pnpm lint — clean
• pnpm test — 171 passing (Vitest + jsdom, incl. StrictMode double-mount + duplicate-name ownership)
• pnpm build