fix(isISO8601): validate midnight second separators

[wangxpych]

Fixes #2858.

The 24:?00 alternative bypassed the minute-separator capture that the seconds backreference uses. This made T24:00:00 fail while allowing nonzero seconds and mixed basic/extended separators after hour 24.

This change moves hour 24 through the same minute-separator capture as other hours, while lookaheads keep its minute and second fields at zero. Regression coverage includes extended and basic midnight forms, nonzero seconds, and separator mismatches under the default, strict, and strict-separator paths.

Validation:

• npm test — 323 passing; all builds and ESLint included

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (not applicable; the public API is unchanged)
• Tests written
• References provided (the existing T24:00 contract and linked issue reproduction cover the behavior)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a79ff98) to head (43c257e).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2872   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2599      2599           
  Branches       658       658           
=========================================
  Hits          2599      2599           

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