test(format): expand date-time suite to 132 exhaustive RFC 3339 cases

[AcEKaycgR]

Addresses #965. This PR expands the date-time format test suite from 29 to 132 cases across all drafts.The expansion is designed to provide complete coverage of RFC 3339 §5.6 ABNF and §5.7 prose constraints, serving as a standalone compliance target for implementors.

Technical Coverage Details

The new cases cover every structural dimension of the grammar:

• Separator validation: T absent, space/tab/duplicate instead of T; colons absent/replaced/duplicated in time; hyphens absent/replaced in date.
• Offset forms: Z-only, numeric offsets (+/-HH:MM), missing offset, offset without colon (+0000), Z followed by numeric offset, offset sign variants (±, ++, +-, -+), non-padded and 3-digit offset fields, and non-ASCII digits.
• Year boundaries: 3-digit, 5-digit, characters just outside ASCII digit range (/ and :), non-ASCII Bengali digits, negative prefixes, and year 0000 (valid) and 9999 (valid) limits.
• Month/Day logic: Full Gregorian calendar coverage—including non-century leap (1996), century/400 leap (2000), century non-leap (1900), and year-0000 leap—month-specific maximums (Jan 31/Apr 30/Feb 28), and day 00 rejection.
• Time & Precision: Floor/ceiling valid values, non-padded fields, and 3-digit hour/minute/second fields; secfrac validation covering 1-digit, 19-digit (arbitrary precision), all-zeros, and non-ASCII mid-frac characters.
• Leap seconds: Confirmed 2016-12-31 IERS date and offset scenarios that produce incorrect UTC.
• Whitespace & Composites: Rejection of empty strings, leading/trailing spaces, tab separators, and trailing garbage.

Standards & Traceability

Following the style established in the ipv4.json suite[cite: 41], this PR adds a top-level comment field citing the full RFC 3339 §5.6 ABNF and §5.7 prose rules. Additionally, it corrects one existing description: "an invalid date-time past leap second, UTC" is updated to "second 61 is above the absolute maximum of 60".

Triangulation Results

The suite was validated against the following implementations:

• Ajv (v8 + ajv-formats): 129 Pass / 3 Mismatch
• Python jsonschema: 126 Pass / 6 Mismatch
• validator.js: 118 Pass / 14 Mismatch

These mismatches represent implementation differences from strict RFC 3339 compliance rather than errors in the test vectors.

Feedback on the coverage matrix and test structure is appreciated @jviotti @jdesrosiers

[karenetheridge]

Please respect the existing formatting of these tests and avoid making unnecessary whitespace changes.

[AcEKaycgR]

Thanks for the review, @karenetheridge.

All three points have been addressed in the latest commit:

• Comment: Trimmed down to a concise prose summary; no more full ABNF quote inline.
• Year 9999 tests: Removed both the "year 9999 is valid" case and the "maximum boundary" composite that also used 9999, for the same timezone/DST reason.
• Indentation: Restored to 4 spaces to match the rest of the suite. For future PRs ,I’ll make sure to respect and match the existing file formatting rather than reformatting on write.