test(server): make Grok flaky-test timeout a hard boundary

[patroza]

Summary

• observe detached Grok test attempts with Fiber.await instead of joining them
• keep the five-second timeout, one retry, warning, and pass-on-failure policy
• request cleanup in a detached interrupt without waiting on wedged finalizers

Why

The prior mitigation still used Fiber.join. Timing out that join propagated interruption into the provider test fiber and waited for its stuck finalizers, so integration CI could still hit Vitest’s global 120-second timeout. Awaiting the detached fiber’s Exit keeps observation independent from cancellation.

Verification

• both affected Grok tests pass focused (2 passed)
• server package typecheck passes; only unrelated existing Effect suggestions remain
• an uninterruptible detached Effect.never crosses the same await/timeout/cleanup boundary in 51 ms with a 50 ms timeout