Test a_retry_waits_on_its_registration_and_another_backup_is_refused fails when the first restore finishes before the third request #908

What happens

backup::tests::a_retry_waits_on_its_registration_and_another_backup_is_refused in crates/engine/src/backup/tests.rs failed in the mongodb-rust job of #830's CI on 2026-11-07 (run 42325897438, job 128260989427):

panicked at crates/engine/src/backup/tests.rs:18576:5:
TableAlreadyExistsException("Table already exists: MusicRestored")

The same job passed twice on main at 76b9710 and once on #411. #830 does not change crates/engine.

Why

The test sends three restore requests for the target MusicRestored:

1. claim_restore_delay holds the first request's registration for 400 ms.
2. A retry of the same request waits for that registration and gets the CREATING description.
3. A restore of another backup into the same name expects TableInUseException. A committed registration gives that answer while its restore job runs.

Nothing holds the first restore's job, and the backup has two items. If the job finishes before the third request, the table is ACTIVE, and the third request gets TableAlreadyExistsException. That is the answer in the failed run.

Suggested fix

Hold the first restore's bulk load with bulk_load_gate and a closed_gate(), as other tests in the same file do. Open the gate after the assertion on the third request.

Related: #740.