test(coding-agent): drained sliced scan verdict in streaming abort test

- 'aborts when a streamed removed line is absent from current content' assumed the guard's time-sliced 2MB scan finishes within the ticks the simulated stream provides plus two trailing sleeps; on slow CI runners the slice budget covers fewer bytes per tick, so the verdict lands later and the assertion read abortTriggered before the scan completed (run 32868138115).
- Drain macrotasks bounded until the verdict lands, matching the sibling responsiveness test; the contract stays 'absent removed line triggers abort', without the per-machine timing premise.