Replace in-memory Promise in ProxyDeps with a checkAndWait/register/resolve
interface so hosts can back it with a longer-lived store (e.g. UserStore DO)
that survives across McpAgent DO instances. onComplete callback is now
() => void | Promise<void> to support async resolvers.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>