Migrate WebMCP surface to document.modelContext (Promise registration, single-arg execute)

[kashishhora]

Summary

First step of bringing the polyfill and hooks in line with the current WebMCP spec. This PR is the structural surface migration:

• registerTool returns a Promise<undefined> that rejects on invalid input (empty / duplicate / over-128-char or illegal name, missing description or execute, non-serializable inputSchema, non–potentially-trustworthy exposedTo origin). Internal tool-change notifications now fan out to multiple listeners.
• Unregistration is AbortSignal-only. The public unregisterTool is gone — pass { signal } to registerTool and abort it to remove the tool. useMcpTool wires this up on cleanup.
• API moved from navigator.modelContext to document.modelContext (the spec moved the getter to Document). Native detection is document-only; the testing API stays on navigator.modelContextTesting.
• execute / handler are single-argument. The ModelContextClient second argument (and requestUserInteraction) is removed.

These are breaking changes. The version bump, CHANGELOG, and doc updates land in a follow-up PR once the rest of the migration is in.

Test plan

• pnpm typecheck — clean
• pnpm lint — clean
• pnpm test — 163 passing (Vitest + jsdom, incl. StrictMode)
• pnpm build

[naji247]

Version bump?