Add Brunei (ms-BN) mobile phone number validation

[snehal242005]

Added mobile phone number validation for Brunei (locale: ms-BN) as part of the ongoing effort in #1761 to add missing country validations.

Brunei mobile numbers are 7 digits starting with 7 or 8, with an optional +673 country code prefix, per ITU numbering plan references.

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 (ce62eb4) to head (82ac904).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2820   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2587      2587           
  Branches       656       656           
=========================================
  Hits          2587      2587           

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