test(surveys): add unit tests for skipSurvey() function

[sannidhyaroy]

Summary

Adds unit tests for skipSurvey() covering the three cases outlined in #444:

• Recurring survey skip (allows_multiple_responses: true, always_visible: true): verifies only _skipped_timestamp is set
• One-time survey skip: verifies only _skipped is set
• Stale flag cleanup: verifies _skipped is removed when a recurring survey is skipped again

Closes #444

Test plan

• npm run test passes

[Copilot]

Pull request overview

Adds missing unit test coverage for skipSurvey() to prevent regressions in recurring vs one-time survey skip behavior (per #444 / PR #420 context).

Changes:

• Import skipSurvey into the existing surveys utils test suite.
• Add 3 new skipSurvey() unit tests covering recurring skip, one-time skip, and stale flag cleanup.

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[sannidhyaroy]

@aaronbrethorst While working on this, I noticed the mock localStorage in the test setup wasn't coercing values to strings like the real browser API does. I've fixed that as part of this PR but I wanted to flag it in case you'd prefer that as a separate change...?

I didn't want to change it initially as I feared it could break existing tests that rely on its current behavior. But, Copilot is right that the toBeTruthy() makes the test weaker. I reviewed and found that all existing setItem calls pass strings ('true').

Let me know if there's anything to adjust here.