fix(isCreditCard): anchor Mastercard regex

[TisanSmart232]

Fixes the Mastercard provider regex so it has one set of anchors around the whole expression.

Before this change, the first Mastercard branch was anchored only at the start:

/^5[1-5][0-9]{2}|...$/

That allowed strings that start with a valid 51-55 Mastercard prefix to pass the provider regex even when they have extra trailing digits. If the full string also happens to pass the Luhn check, isCreditCard can return true for an invalid Mastercard length.

Before:

validator.isCreditCard('53982287078715277') // true

After:

validator.isCreditCard('53982287078715277') // false

The change is limited to isCreditCard and adds a regression case for both the default validator path and the explicit { provider: 'Mastercard' } path.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

Tests

• npm run lint
• npx mocha --require @babel/register --reporter dot test/validators.test.js --grep "credit cards"
• npm test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3d2f4b3) to head (7597f48).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2788   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2587      2587           
  Branches       656       656           
=========================================
  Hits          2587      2587           

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