test(isBase64): add large-string regression tests

[easedu]

Description

Adds regression test coverage for large base64 strings as reported
in #2573.

Tests added

• ~1MB valid base64 string
• ~1MB invalid base64 string (injected invalid char)
• ~1MB valid base64 string with urlSafe option

All 318 tests passing, 100% statement/line/function coverage.

Checklist

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

Refs #2573

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b1aea75) to head (a962534).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2681   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2595      2595           
  Branches       659       659           
=========================================
  Hits          2595      2595           

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[WikiRik]

I think we already solved this, this addition doesn't make the code more readable for me. This library doesn't need to be very performant, it mostly should not crash on long strings. We can merge the new test if that's useful

[easedu]

Thanks for the review @WikiRik! Happy to simplify this PR to just the new large-string test cases and revert the implementation change.
I'll update shortly.

[easedu]

Updated — reverted the implementation change and kept only the new large-string test cases.

Note: the tests pass here because ~1MB strings are within V8's regex stack limit in most environments. The original issue (#2573) manifests with larger strings or smaller stack limits. The tests still add useful regression coverage.

Let me know if anything else needs adjustment.