[Tests] Make 'refreshes extension url' test deterministic

[gonzaloriestra]

WHY are these changes introduced?

The test refreshes extension url in packages/ui-extensions-server-kit/src/state/reducers/extensionServerReducer.test.ts was skipped and relied on real-time passage (setTimeout) to verify timestamp changes. This is non-deterministic and can lead to flakiness.

WHAT is this pull request doing?

• Unskips the refreshes extension url test.
• Refactors the test to use vi.useFakeTimers() and vi.setSystemTime().
• Replaces loose assertions with exact timestamp checks.
• Removes the asynchronous setTimeout block, making the test synchronous and faster.

How to test your changes?

Run the unit tests for the affected package:

pnpm --filter @shopify/ui-extensions-server-kit vitest run src/state/reducers/extensionServerReducer.test.ts

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is NOT user-facing.

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PR created automatically by Jules for task 8839888816816504392 started by @gonzaloriestra

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