Tests: update tests to account for different month durations

[jeherve]

Fixes #2723

Proposed changes:

Update tests to work regardless of the month (see #2723)

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Do the tests pass?

Changelog entry

• Automatically create a changelog entry from the details below.

Changelog Entry Details

Significance

• Patch
• Minor
• Major

Type

• Added - for new features
• Changed - for changes in existing functionality
• Deprecated - for soon-to-be removed features
• Removed - for now removed features
• Fixed - for any bug fixes
• Security - in case of vulnerabilities

Message

[Copilot]

Pull request overview

This PR fixes a flaky test issue in test_purge_ap_posts_more_than_200_posts caused by month duration variations. The test was using -1 month to create posts that should be kept (newer than 30 days), but this could produce dates older than 30 days when run on certain days of the month (e.g., January 31st producing December 31st).

Key changes:

• Updated comments to accurately reflect the 30-day purge threshold (was incorrectly documented as "6 months")
• Changed date for posts that should be kept from -1 month to -1 week to ensure they are always within the 30-day threshold

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/phpunit/tests/includes/class-test-scheduler.php	Fixed flaky test by updating comments to reflect accurate 30-day threshold and changing test date from -1 month to -1 week to avoid month boundary issues
.github/changelog/fix-purge-ap-posts-test-date-boundary	Added changelog entry documenting the test fix

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