bug(core): AbortSignal is ignored when source resolution has no timeout

[vamgan]

Summary

Abort cancellation for asynchronous context-source resolution is ignored unless timeoutMs is also provided.

Root cause

packages/core/src/context.ts, runSourceTask(), returns the source promise immediately when timeoutMs === undefined. The AbortSignal listener is only installed in the timeout branch.

A pending source resolved with { signal } but no timeout therefore remains pending after the caller aborts.

Proposed fix

Race against abort whenever a signal is supplied, independently of timeout handling. Remove abort listeners on every success/failure/timeout path.

Acceptance criteria

• Aborting a pending source without a timeout rejects promptly
• Abort with a timeout still works
• Already-aborted signals remain rejected immediately
• Successful and failed tasks remove signal listeners
• No timer is created when no timeout is configured