fix(mock): settle the dispatch when an async reply data function rejects

[pacocartones]

What

MockPool/MockAgent .reply(statusCode, dataFn) supports an async data function — the returned promise is already awaited in handleReply, and it's exercised in the existing suite (test/mock-agent.js:578, :3487). But that awaited promise had only a fulfilment handler: if the data function rejects, no onResponse* callback ever fires, so the dispatched request never settles (hang) and the rejection escapes as an unhandledRejection.

Fix

Add the rejection arm, mirroring the file's own conventions — the sibling reply-options-callback path already handles both fulfil and reject, and the sync path re-checks aborted:

return body.then(
  (newData) => handleReply(mockDispatches, { ..._response, data: newData }),
  (err) => {
    if (aborted) return
    deleteMockDispatch(mockDispatches, key)
    handler.onResponseError?.(controller, err)
  }
)

The request now rejects with the error, exactly one terminal callback, no unhandled rejection. Sync/options paths are untouched.

Test

test/mock-async-reply-reject.js: .reply(200, async () => { throw boom }) then a request, with a timeout guard and an unhandledRejection listener. RED on main (unhandled rejection; request never settles). GREEN with the fix (rejects with boom, data fn called once, no unhandled rejection).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.49%. Comparing base (17117f6) to head (67a405b).

Files with missing lines	Patch %	Lines
lib/mock/mock-utils.js	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5762      +/-   ##
==========================================
- Coverage   93.50%   93.49%   -0.01%     
==========================================
  Files         110      110              
  Lines       39072    39084      +12     
==========================================
+ Hits        36534    36543       +9     
- Misses       2538     2541       +3     

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