fix(isISO8601): validate years before 1000 in strict mode

[DruckerE]

Description

Fixes #2517.

When strict: true, isISO8601 rejected otherwise-valid dates whose year is below 1000 (for example 0001-01-01, 0001-01-13T00:00:00.000Z, 0099-12-31).

Root cause

In isValidDate (src/lib/isISO8601.js) the matched year is cast to a Number:

const match = str.match(/(\d{4})-?(\d{0,2})-?(\d*)/).map(Number);
const year = match[1]; // "0001" -> 1

The ISO 8601 regex guarantees a 4-digit year, but the Number cast drops the leading zeros. The date string was then rebuilt as `${year}-${monthString}-${dayString}` → 1-01-01, which new Date() misparses, so the d.getUTCFullYear() === year round-trip check failed and the date was reported invalid.

Fix

Pad the year back to 4 digits before building the Date, mirroring the existing month/day padding:

const yearString = `000${year}`.slice(-4);
const d = new Date(`${yearString}-${monthString || '01'}-${dayString || '01'}`);

Tests

Added valid (0001-01-01, 0001-01-13T00:00:00.000Z, 0099-12-31, 0500-06-15) and invalid (0001-13-01, 0001-01-32) cases to the existing strict = true suite. Full suite passes (318 passing).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3d2f4b3) to head (f76d789).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2779   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2587      2588    +1     
  Branches       656       656           
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+ Hits          2587      2588    +1     

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