fix(isCreditCard): anchor mastercard regex so partial strings are rejected

[LorenzoGalassi]

Closes #2717

Problem

isCreditCard("5108") returns true, and so do many other short or partial strings beginning with 51-55.

The mastercard pattern is:

mastercard: /^5[1-5][0-9]{2}|(222[1-9]|22[3-9][0-9]|2[3-6][0-9]{2}|27[01][0-9]|2720)[0-9]{12}$/

| binds looser than the ^ and $ anchors, so this is really the union of two half-anchored patterns:

1. ^5[1-5][0-9]{2} — start-anchored only, matches any string starting with 51XX-55XX regardless of length.
2. (222[1-9]|...|2720)[0-9]{12}$ — end-anchored only.

So format validation degenerated to "starts with 51-55" or "ends with a 2-series BIN + 12 digits". "5108" matches branch 1 and also happens to be Luhn-valid, so isCreditCard returned true.

Fix

Wrap the alternation in a non-capturing group so both anchors and the trailing [0-9]{12} apply to every branch:

mastercard: /^(?:5[1-5][0-9]{2}|222[1-9]|22[3-9][0-9]|2[3-6][0-9]{2}|27[01][0-9]|2720)[0-9]{12}$/

All Mastercard numbers are 16 digits (4-digit BIN prefix + 12), which the grouped pattern now enforces for both the 5x and 2x ranges.

I audited the other six card patterns in this file for the same |-precedence issue; only mastercard was affected. (unionpay looks unusual but its branches are correctly wrapped in ^(...)$, so it is not impacted.)

Tests

Added 5108 to the invalid lists in both the Mastercard-provider and no-provider isCreditCard test blocks.

• npm run build + full validators.test.js suite: 249 passing.
• Verified the new cases are meaningful: reverting only the regex change makes exactly those two assertions fail (isCreditCard("5108") and isCreditCard("5108", { provider: "Mastercard" })) and nothing else.
• Valid Mastercards (both 51-55 and the 2221-2720 2-series BINs) continue to validate; "5108", "5108foo" now correctly return false.
• eslint src test passes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fdc788) to head (f247e1a).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2747   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2587      2587           
  Branches       656       656           
=========================================
  Hits          2587      2587           

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