fix(test): stabilize flaky useReplaysFromIssue assertion test

[JoshuaKGoldberg]

Removes synchronous initial-state assertions (isFetching: true) from useReplaysFromIssue hook tests. These assertions are flaky because renderHookWithProviders uses act() internally, which flushes effects. Since MockApiClient resolves synchronously, the hook's useEffect → fetchReplayIds() → setReplayIds() chain can complete within the same act() boundary, leaving the hook already resolved by the time the synchronous assertion runs.

The waitFor assertions that check the final resolved state remain. I think they're enough to verify the hook's behavior.
(I actually think and wrote that, not Cursor)

Fixes ENG-7207

Made with Cursor

[linear-code]

ENG-7207 Flaky test: static/app/views/issueDetails/groupReplays/useReplaysFromIssue.spec.tsx

[github-actions]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

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[sentry]

🚧 Skipped: PR exceeds review size limit.

Please split into smaller PRs and re-run.
_{Reference ID: 12937434}