core: fix unhandled errors when aborting with queued messages

[thdxr]

Summary

• Fixes unhandled promise rejection errors that occur when aborting a session that has a queued message waiting
• The issue was introduced in 6c9b2c3 which changed queued callback rejections from reject() to reject(new DOMException("Aborted", "AbortError")) — callers like prompt_async don't await/catch the returned promise, so the rejection propagated as unhandled errors
• Instead of rejecting queued callbacks on abort, we now simply drop them — the promises are GC'd and the already-persisted user messages remain in history for the next interaction