test: cover scheduled script navigation

[Tiancheng-Xu]

Summary

• Add deterministic regression coverage for a main-frame navigation scheduled by a script after the action returns.
• Exercise the waitForStableDom: false path from issue "still awaits a navigation when waitForStableDom is false" is flaky #2609.

The production navigation probe is already present on main from #2622; this PR adds the missing regression coverage.

Validation

• npm run check-format
• npm run test tests/utils/WaitForHelper.test.ts
• npm run test tests/tools/script.test.ts

Fixes #2609,

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[nattallius]

Hi, thank for your pr, can you please sign the CLA

[Lightning00Blade]

Closing as stale.