Update error handling in the MockPool example to reflect the correct error type and provide additional error context.

When a MockAgent had a delayed response and the request was aborted via
AbortSignal, the delayed response callback would still fire and attempt to
call handler methods after onError had been called. This violated state
invariants expected by handlers like DecoratorHandler (used by decompress
interceptor).

The fix:
- Tracks abort state in mockDispatch
- Clears pending setTimeout timer when abort is called
- Checks abort flag in handleReply before invoking handler callbacks
- Properly calls handler.onConnect to register abort callback

Fixes #4693

In cases where onParserTimeout is called with a weak reference that has
already been collected it was throwing an error like:

    Cannot destructure property 'socket' of 'parser.deref(...)' as it is
    undefined

- Fixes #4755

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>