Stabilize git metadata FSEvents watcher - #5131

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@cmuxTests/WorkspacePullRequestSidebarTests.swift`:
- Around line 1646-1650: The assertion currently checks XCTAssertLessThan(delay,
1.0) which equals the waitTimeout and leaves no margin for CI jitter; change the
asserted bound to a value comfortably below the timeout (for example assert
delay < 0.8) and ensure the helper's waitTimeout remains higher than that bound
(e.g., keep waitTimeout at 1.0 or increase it to 1.2) so the measured delay and
XCTUnwrap have headroom; update the assertion call (XCTAssertLessThan with
variable delay) and, if necessary, the waitTimeout passed into the helper used
to measure delay so the test is robust against timing jitter.
- Around line 1635-1651: The assertion in
testGitMetadataWatcherRefreshesDuringSustainedEventStorm uses
XCTAssertLessThan(delay, 1.0) which is exactly the waitTimeout and can be flaky;
update the assertion in this test
(testGitMetadataWatcherRefreshesDuringSustainedEventStorm) to use a safer
threshold (e.g., XCTAssertLessThan(delay, 0.9) or 0.95) when checking the value
returned by
TabManager.workspaceGitMetadataWatcherFirstRefreshDelayDuringStormForTesting(...)
so the test has slack against CI timing jitter.

In `@Sources/TabManager.swift`:
- Around line 5022-5065: The helper is nondeterministic because it starts timing
before creating the watcher, uses NSTemporaryDirectory() (sharing unrelated FS
events), and leaves the background producer running after timeout; fix by
creating a unique temporary directory (e.g.,
FileManager.temporaryDirectory.appendingPathComponent(UUID().uuidString)) and
pass that into WorkspaceGitMetadataWatcher.Descriptor instead of
NSTemporaryDirectory(), start the startedAt clock only after createdWatcher is
non-nil (i.e., after watcher is constructed), and run the producer as a
cancellable DispatchWorkItem (store it, call workItem.cancel() in the defer that
calls watcher.stop()) so the loop checks workItem.isCancelled and exits to avoid
enqueueing events after the helper returns; ensure semaphore.wait timeout still
returns nil as before.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In @.github/workflows/test-depot.yml:
- Around line 142-143: Update the workflow input description for
inputs.test_filter to clearly state that the filter applies to both unit and UI
tests (it sets the TEST_FILTER env var used in the unit test job and is also
applied to the UI test job), so users know a single value will filter both test
runs; locate the input named test_filter and change its description to mention
both "unit and UI tests" and that it sets TEST_FILTER (inputs.test_filter ->
env: TEST_FILTER) so the intent matches its use in the jobs (including the UI
test step that reads the same input).
- Around line 148-158: The unit-test invocation fails under bash -u because
ONLY_TESTING is an empty array and "${ONLY_TESTING[@]}" expansion can be treated
as an unbound variable on older macOS bash; change the logic to use a string arg
instead of an empty array: initialize ONLY_TESTING_ARG="" and if [ -n
"$TEST_FILTER" ]; then ONLY_TESTING_ARG="-only-testing:cmuxTests/$TEST_FILTER";
fi, then pass "$ONLY_TESTING_ARG" into the xcodebuild invocation (replace
"${ONLY_TESTING[@]}" with "$ONLY_TESTING_ARG"); alternatively, keep the array
but use a safe expansion pattern like "${ONLY_TESTING[@]:-}" to avoid the
unbound variable error.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In @.github/workflows/test-depot.yml:
- Around line 148-157: The workflow assumes expanding ${ONLY_TESTING[@]} is safe
under all runner shells, but the reviewer noted the failure claim applies only
to Bash 3.2; instead make the invocation robust regardless of shell by
conditionally appending the array only when it contains elements. Update the
block that builds and calls xcodebuild (symbols: ONLY_TESTING, TEST_FILTER,
scripts/ci/xcodebuild_noninteractive.py, xcodebuild ... test) so you check if [
"${`#ONLY_TESTING`[@]}" -gt 0 ] and, if true, include "${ONLY_TESTING[@]}" in the
command; otherwise call the same command without that expansion—this removes
dependence on shell-specific empty-array expansion semantics and avoids
unbound-variable problems under set -u.