fix(isISO8601): validate signed ordinal dates in strict mode

[abhijeet117]

Summary

In strict mode, isValidDate matched ordinal dates with a pattern that had no sign prefix, even though the main isISO8601 patterns accept a leading + or - on the year. Signed ordinal dates therefore fell through to the calendar-date branch, where the three-digit day of year was split into a bogus month and day: valid dates like +2009-145 were rejected, and others passed without any day-of-year check. The pattern now accepts an optional sign and routes these to the existing ordinal validation, including its leap-year bound.

Testing

Reproduced before the fix: isISO8601('+2009-145', { strict: true }) returned false while the unsigned 2009-145 returned true. Added a focused case in test/validators.test.js covering signed valid dates, a leap-year day 366, and a non-leap day 366 rejection.

Checklist

• bug reproduced before fix
• root cause identified
• bug fixed
• tests passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a79ff98) to head (917c3c8).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2865   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2599      2599           
  Branches       658       658           
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  Hits          2599      2599           

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