MainActor.assertIsolated() requires Swift 5.9 (Xcode 15); the macOS 12
CI runner compiles the test bundle with Swift 5.7 and failed run 31985.
Assert Thread.isMainThread instead — it checks the same contract (the
coalesced log sink hops back to the main actor).
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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>