Fix: org party regime validation

[samlown]

Describe your changes

Fixes an issue validating tax ID codes when the regime is provided inside an org.Party.

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added thorough tests with at least 90% code coverage.
• I've modified or created example GOBL documents the show my changes in use, if appropriate.
• When adding or modifying a tax regime or addon, I've added links to the source of the changes either structured or in the comments.
• I've run go generate . to ensure that Schemas and Regime data are up to date.
• All linter warnings have been reviewed and fixed.
• I've been obsessive with pointer nil checks to avoid panics.
• The CHANGELOG.md has been updated with an overview of my changes.
• Requested a review from @samlown.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.64%. Comparing base (eb4879b) to head (956b2e3).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #434      +/-   ##
==========================================
+ Coverage   81.63%   81.64%   +0.01%     
==========================================
  Files         235      235              
  Lines       11539    11546       +7     
==========================================
+ Hits         9420     9427       +7     
  Misses       1756     1756              
  Partials      363      363              

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