Fix flaky fetch call-count assertions in MiddlewareTest

[duongynhi000005-oss]

Summary

Stabilize two flaky HandleUnusedOptimisticID test cases by waiting for global.fetch to reach the expected call count before asserting.

Changes

• Added waitForFetchCallCount() helper in tests/unit/MiddlewareTest.ts.
• Replaced direct toHaveBeenCalledTimes(2) assertions in the two flaky tests with await waitForFetchCallCount(2).

Testing

• Not run locally in this environment.

$ #90660

[github-actions]

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