Fix: correct French VAT (FR) validation regex and add tests

[iamAmer]

Fixes #2582

Summary

This PR fixes the French VAT number validation.

Changes

• Updated the regex for FR VAT numbers to only allow the correct format: FR + 2 digits + 9 digits (SIREN).
• Updated/added test cases to match the correct specification.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (eee525c) to head (1fa4aa4).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2584   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2536      2536           
  Branches       642       642           
=========================================
  Hits          2536      2536           

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[WikiRik]

For future improvement we might want to look at some leniency for whitespaces in between the 'segments' (so something like FR 12 345678912 might be valid as well) but on a quick search I didn't find good resources for that. For now this is already an improvement.

I do also see all resources pointing towards requiring FR at the start, so that's also a change I'm approving

[iamAmer]

Thanks for the review! I agree — adding leniency for whitespaces could be a useful future improvement. For now, I’ll leave it strict since that’s what most resources confirm.