Enhance validation coverage for string formats with robust and portable edge cases

[OmSwainSupreme]

Problem

Several string-based formats lacked comprehensive validation for actual string values and edge cases.
Most existing tests primarily focus on non-string inputs, leaving gaps in real-world validation coverage.

Solution

This PR enhances validation coverage by adding detailed test cases for multiple string formats, including:

• email
• regex
• ipv4
• hostname
• date
• date-time
• time
• uri
• uuid

Improvements

• Added both valid and invalid edge cases for each format
• Ensured all test cases are portable and not implementation-dependent
• Replaced ambiguous patterns (e.g., regex and URI cases) with clearly invalid, standards-compliant examples
• Maintained consistency with existing test structure and style

Impact

Improves robustness, correctness, and cross-implementation consistency of the JSON Schema test suite.

Notes

Care was taken to avoid implementation-specific behavior and ensure compatibility across different validators.

[karenetheridge]

The format.json file at the main level of the tests is for the format-annotation library, where all strings should validate, and all non-strings should also validate because they are ignored. There are comprehensive tests for the format-assertion library in the optional/format directories.

Please familiarize yourself with the layout of this test suite and the standards we follow, by consulting its documentation e.g. README.md, before submitting further patches, and propose your changes in an issue first.