Stabilize AppServer lifecycle interrupt join test

[lynnswap]

Purpose

Remove a scheduler-dependent terminal assertion that intermittently fails the AppServer lifecycle test and blocks unrelated pull requests.

Changes

• Keep the event-session setup used by the lifecycle close test.
• Keep the assertions proving close remains incomplete while the admitted interrupt is held and completes only after that interrupt finishes.
• Remove the unrelated assertion that raced notification-stream failure against the successful interrupt terminal; dedicated tests continue to own router and session failure propagation.

Testing

• Targeted test repeated 1,000 times
• AppServer tests (206 tests)
• swift test --build-system swiftbuild --no-parallel
• scripts/check-compatibility.sh
• ReviewMonitor xcodebuild test (15 tests)
• Two independent exact-diff audits

Screenshots

Not applicable.

[lynnswap]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 👍

Reviewed commit: 8ffd64f4ee

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