waitForManagedBashJob raced job completion against a bare
Bun.sleep(thresholdMs), which cannot be cancelled. When completion,
abort, or steering won the race, the losing Bun.sleep timer stayed
scheduled and ref'd, keeping Bun's event loop alive until the threshold
expired — delaying SDK/headless shutdown and accumulating timers under
fast command rates.
Replace the Bun.sleep with a Promise.withResolvers settled by a
cancellable setTimeout, and route every outcome (including the former
no-signal early return) through one try/finally that clears the timer
and removes the abort/steer listeners.
Add a child-process regression test that runs the real auto-background
path for a fast command against a 30s threshold and asserts the process
exits promptly instead of being held for the full threshold.
Fixes#7235
roboomp