en16931: validate party identity scope cardinality

[alvarolivie]

Summary

Adds two validation rules to the EN 16931 addon's org.Party identities, mirroring the standard's single-valued registration identifiers:

• At most one identity may carry the legal scope — the legal registration identifier (BT-30 seller / BT-47 buyer).
• At most one identity may carry the tax scope — the VAT identifier (BT-31 seller / BT-48 buyer).

Both rules share a small orgIdentitiesScopeCount helper. A party carrying zero such identities stays valid; only duplicates are rejected. go generate . regenerated data/rules/eu-en16931.json with the two new fault codes. Validation-only change — no normalization/schema output shift, and the examples suite passes unchanged, so no example document needed updating.

Pre-Review Checklist

• Opened this PR as a draft
• Read the CONTRIBUTING.md guide.
• Performed a self-review of my code.
• Added thorough tests with at least 90% code coverage.
• Modified or created example GOBL documents to show my changes in use, if appropriate.
• Added links to the source of the changes in tax regimes or addons, either structured or in the comments.
• Run go generate . to ensure that the Schemas and Regime data are up to date.
• Reviewed and fixed all linter warnings.
• Been obsessive with pointer nil checks to avoid panics.
• Updated the CHANGELOG.md with an overview of my changes.
• Marked this PR as ready for review.

And if you are part of the org:

• Requested a review from Copilot and fixed or dismissed (with a reason) all the feedback raised.
• Requested a review from @samlown.

[Copilot]

Pull request overview

Adds EN 16931 validation to ensure org.Party.Identities does not contain duplicate registration identifiers for the standard’s single-valued party identifiers (legal registration and VAT/tax), aligning with BT-30/47 and BT-31/48 cardinality.

Changes:

• Add EN 16931 validation rules enforcing “at most one” legal scoped identity and “at most one” tax scoped identity on org.Party.Identities.
• Introduce a small helper to count identities by scope and reuse it across both validations.
• Add unit tests covering allowed vs. duplicate scope cases, and document the behavior in the changelog.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
CHANGELOG.md	Documents the new EN 16931 party identity scope cardinality validation.
addons/eu/en16931/org.go	Adds the two scope cardinality assertions and a shared helper for counting scoped identities.
addons/eu/en16931/org_test.go	Adds tests verifying both legal-scope and tax-scope “at most one” enforcement.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.78%. Comparing base (8b0bc06) to head (2a4567b).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #899      +/-   ##
==========================================
+ Coverage   94.76%   94.78%   +0.01%     
==========================================
  Files         318      319       +1     
  Lines       17269    17360      +91     
==========================================
+ Hits        16365    16454      +89     
- Misses        559      560       +1     
- Partials      345      346       +1     

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

[samlown]

Quick check on the boolean responses required.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.