de/Enhance date test coverage with special cases

[coderabbitai]

This issue was created based on this comment on PR #1 by @JarbasAl to track enhancing date test coverage with special cases.

ovos_date_parser/res/de-de/date_time_test.json

Enhance date test coverage with special cases.

Consider adding test cases for:

Leap year dates (February 29th)
Month/year boundaries (December 31st, January 1st)
Recent years (2023-2024)
Special day formatting (1st, 2nd, 3rd vs other ordinals)