test: fix flaky cleanup, schema-apply and admission tests

[azimafroozeh]

What & why

Fix flaky cleanup, schema-apply and admission tests. The failures come from shared failpoints and a sleep that assumes all admission attempts finish within 50 ms.

• Hold FailScenario throughout cleanup and schema-apply tests so sibling tests cannot inject faults into them.
• Replace the admission test's 50 ms sleep with a barrier, retaining guards until all 32 attempts complete.

Backing issue / RFC

• Fixes an accepted issue: none linked.
• Is an RFC PR, or implements an accepted RFC: none linked.
• Trivial fast-lane.

Prompted by the cleanup test failure on main.

Checklist

• Change is focused (test isolation and synchronization).
• Tests added/updated for behavior changes (existing cleanup, schema-apply and admission tests updated).
• Public docs updated if user-facing surface changed (N/A, test-only changes).
• Reviewed against docs/dev/invariants.md (no production logic or storage behavior changed).

Local verification

• cargo test -p omnigraph-engine -p omnigraph-server --lib --locked --features omnigraph-engine/failpoints --quiet: 556 passed, 3 ignored.
• cargo check --workspace --locked -q --message-format=short: passed.
• cargo clippy -p omnigraph-engine -p omnigraph-server --lib --tests --locked --features omnigraph-engine/failpoints -q --message-format=short -- -D warnings -W clippy::dbg_macro: passed.
• cargo fmt --all --check: passed.
• git diff --check HEAD: passed.
• Repeated compiled test groups with 2 and 32 test threads: cleanup 100/100 runs, schema application 100/100, admission 300/300.

Notes for reviewers

Existing assertions remain unchanged. The guards cover each test's full operation lifetime; the admission barrier releases only after every attempt completes.