fix: handle day overflow in monthly interval increment

[petersalomonsen]

Problem

The monthly interval increment function would panic when incrementing from dates like January 31st to February, because February doesn't have 31 days.

Solution

Use chrono's checked_add_months method which automatically handles day overflow by clamping to the last valid day of the target month.

Changes

• Updated Interval::increment() to use checked_add_months(Months::new(1))
• Added comprehensive unit tests covering edge cases:
  • Jan 31 → Feb 29 (leap year)
  • Jan 31 → Feb 28 (non-leap year)
  • Mar 31 → Apr 30
  • May 31 → Jun 30
  • Jan 29-30 → Feb (both leap and non-leap years)

Testing

All 13 unit tests pass:

cargo test interval_increment --lib

[Copilot]

Pull request overview

This PR fixes a panic in the monthly interval increment function when handling end-of-month date overflow scenarios (e.g., January 31st → February). The solution uses chrono's built-in checked_add_months method which automatically clamps to the last valid day of the target month.

Key Changes:

• Replaced manual month/year boundary handling with chrono's checked_add_months(Months::new(1))
• Added comprehensive unit tests covering 13 edge cases including leap years, various month-end dates, and year boundaries
• Updated documentation to clarify day overflow clamping behavior