Codex SG tax id validation

[methodofaction]

Summary

• Validate Singapore UEN check characters for business, local-company, and other-entity formats.
• Restrict other-entity UENs to known entity types.
• Accept IRAS-assigned GST registration numbers with digit suffixes.
• Update Singapore examples, generated digests, changelog entries, and test fixtures.

Testing

• Expanded Singapore identity and tax identity test coverage for valid and invalid UENs and GST numbers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.01%. Comparing base (8652979) to head (596d067).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #933      +/-   ##
==========================================
+ Coverage   94.99%   95.01%   +0.01%     
==========================================
  Files         327      327              
  Lines       17732    17736       +4     
==========================================
+ Hits        16845    16851       +6     
+ Misses        547      546       -1     
+ Partials      340      339       -1     

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