fix: use stable test dates in duplicate API calls E2E tests

[keithwillcode]

What does this PR do?

Fixes flaky E2E tests in booking-duplicate-api-calls.e2e.ts that were failing on December 31 due to month boundary issues.

The tests were using new Date() and then setting the day to 5 or 20, which caused issues when the prefetch logic in isMonthViewPrefetchEnabled evaluated the current date against month boundaries. On certain dates (like end of month/year), this could trigger additional API calls, causing the tests to fail with "Expected <= 1, Received 2".

This PR introduces a getStableTestDate() helper function that returns a fixed future date (July 2030) to ensure consistent test behavior regardless of when the tests are run.

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A - test-only change.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

1. Run the E2E tests: PLAYWRIGHT_HEADLESS=1 yarn e2e booking-duplicate-api-calls.e2e.ts
2. All 6 tests should pass regardless of the current date

Human Review Checklist

• Verify that using a fixed future date (July 2030) doesn't change the fundamental behavior being tested
• Confirm the hypothesis that month boundary prefetch logic was causing the extra API calls
• Consider if there's a more robust approach (e.g., more comprehensive date mocking)

Note: Local testing was blocked by environment issues (Internal Server Error), so this fix is based on code analysis of the prefetch logic in isMonthViewPrefetchEnabled.ts. CI will verify the fix.

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Link to Devin run: https://app.devin.ai/sessions/b1ce96e74aa14104aa9eb14f5e3449f6
Requested by: keith@cal.com (@keithwillcode)

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