Add tests for get_calendar() and validate input parameters

[pbearne]

Introduced comprehensive PHPUnit tests to cover various scenarios for the get_calendar() function. Enhanced input validation in get_calendar() to handle invalid week numbers and months, defaulting them to safe values when out of range.

Trac ticket: https://core.trac.wordpress.org/ticket/41011

[github-actions]

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[github-actions]

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[mukeshpanchal27]

Quick review.

Add @ticket annotation for all the new unit tests.