[bahriy-dev]

isVAT(str, 'TR') currently accepts any 10-digit string. The Turkish tax number (Vergi Kimlik Numarası, VKN) has a check digit, so numbers such as 1234567891 or 9876543210 should be rejected.

This keeps the existing format (optional TR prefix + 10 digits) and adds the check digit calculation. The existing valid test values still pass; new invalid cases fail on master and pass with this change.

I also ran the implementation against the 200 VKNs in python-stdnum's test file; all of them validate.

References:

• python-stdnum implementation: https://github.com/arthurdejong/python-stdnum/blob/master/stdnum/tr/vkn.py
• Official lookup service: https://www.turkiye.gov.tr/gib-intvrg-vergi-kimlik-numarasi-dogrulama

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 (9ff3424) to head (b7f46f6).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2960   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2599      2609   +10     
  Branches       658       661    +3     
=========================================
+ Hits          2599      2609   +10     

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