feat(isMobilePhone): add support for Guinea (fr-GN)

[rezk2ll]

Summary

• Add mobile phone validation for Guinea (locale: fr-GN)

Details

• Country code: +224
• Mobile numbers start with 6 followed by 8 digits
• Total: 9 digits after country code
• Pattern: /^(\+?224)?6\d{8}$/

Reference

Telephone numbers in Guinea

Checklist

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (c3dc37c) to head (1c3f0cd).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2662   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2572      2572           
  Branches       651       651           
=========================================
  Hits          2572      2572           

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