fix(64450): clear the pending program update timer when a watch program is closed

[DevPadawan (skywalkersPadawan)]

Fixes #64450

Analysis

createWatchProgram().close() did not clear the pending
timerToUpdateProgram.

When a watched file changed shortly before close(), the 250 ms
program-update timer could still fire after the watch program was closed.
The callback could then synchronize the program and recreate file
watchers, leaving the Node.js process alive.

Fix

Clear timerToUpdateProgram in createWatchProgram().close() before
clearing the resolution cache and closing the remaining watchers.

Added a watchAPI regression test that changes a watched file, closes
the watch program before the queued update runs, drains the queued
timeout callbacks, and verifies that the pending update is cleared and
the watchers remain deleted.

Validation

• Targeted regression test: npx hereby runtests --tests="clears the pending program update" — 1 passing
• watchAPI suite: npx hereby runtests --tests="watchAPI" — 43 passing
• npx hereby local — passed
• npx hereby runtests-parallel — 106,371 passing
• Regression test and baseline added
• Working tree clean after validation

runtests-parallel also reported an unrelated lint error in an
untracked generated file under tsc/testdata/fixtures/compiler/.
The generated tsc/ directory was removed afterward and is not part
of this PR.

AI assistance

ChatGPT was used for investigating the issue, identifying the relevant code paths, and finding the relevant files to work on. Cursor was used with its review agent to perform an implementation check. I manually implemented the fix and regression test, reviewed all resulting diffs, and understood the implementation. I would be happy to discuss the changes and provide additional details about the fix if needed.

[typescript-automation]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

[DevPadawan (skywalkersPadawan)]

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[DevPadawan (skywalkersPadawan)]

@microsoft-github-policy-service agree

[Jake Bailey (jakebailey)]

We're definitely not going to be backporting this or releasing 6.0.