Fix a crash caused by improper management of async workers

[savetheclocktower]

This is the cause of at least one of the crashes reported in pulsar#1438, but hopefully all three.

The theory of the crash, produced during a rubber-ducking session with Claude:

• TextBuffer::findWordsWithSubsequence is called (this is used by autocomplete-plus’s built-in subsequence provider) and delegates tofind_words_with_subsequence_in_range in the C++ code
• that can be a costly task, so it's async; a job is scheduled and placed in outstanding_workers
• a subsequent call to TextBuffer::setTextInRange calls set_text_in_range in the C++; this can invalidate whatever data find_words_with_subsequence_in_range was collecting, so it cancels the worker via cancel_queued_workers and worker->Cancel()
• if we're unlucky, the worker is never removed from outstanding_workers because that happens in the worker’s Execute method — and we could've cancelled before that method was called
• later, set_text_in_range triggers another call to cancel_queued_workers
• the already-cancelled job from before is still present in the set
• we try to call Cancel on it again, but the memory has been freed, and everything falls apart

The fix is to take the code that removes a job from outstanding_workers and move it from Execute to OnWorkComplete. The latter is guaranteed to be called, no matter how a job finished. Also, Execute runs in its own thread, but OnWorkComplete runs on the main thread, where all outstanding_workers access should happen in the first place.

Because this is a theory produced by a man and his hallucinating robot, it's important to back it up with proof. The first commit in this PR adds a failing test; the second commit makes the test pass. So if you're reviewing this PR:

• Checkout 1f1caff, run npm run build:node, then npm run test:node and ensure you see a failing test with a crashdump
• Checkout b89da52, repeat these steps, and see a green test suite

[confused-Techie]

While not versed in C, the changes here are extremely minimal, simply moving existing coding in-line with what you've described.

Plus seeing tests added for exactly this, and that they are passing in CI makes me think we are 100% in the clear for this being a resolution.

Just prior to merging make sure to bump the version key in package.json so we can easily make a release from this, and with that change happy to approve and get this merged!