Flaky: test an engine that exits before answering initialize

`apps/jaahd/tests/test_acp_host.py::test_an_engine_that_exits_before_answering_initialize_is_a_crash` fails about half the time on develop: 7 of 12 runs, measured in task 7568. The engine (`sys.exit(3)`) sometimes closes stdout before the host sees the process exit. The break detail then reads `never started a session (init_failed): connection closed` instead of `(engine_exit)`. Ask: make the host classify a stdout close that arrives together with an engine exit as `engine_exit` (for example, wait briefly for the exit code before choosing the cause), or make the test accept the race deterministically. Prefer the product fix, because the operator sees this wording. Out of scope: other ACP host behaviour. Done means: - The test passes 50 consecutive runs. Required checks: ./dc check full

Product fix: the ACP host now waits for the engine's exit when a stdout close ended the turn loop before any session, so it reports engine_exit. The test failed 6 of 12 runs before and passed 50 of 50 after; the quick gate is green. PR #6086 is queued in the merge lane.