Allow item attributes to hold more than one value

[alvarolivie]

• org: rule GOBL-ORG-ATTRIBUTE-02 relaxed from exactly one of text, code, amount, or date to at least one, so an attribute can group related values under a single name. An attribute with no value at all is still invalid.
• Doc comment and data/rules/org.json updated to match. The JSON schema is unchanged, as it only carries the first sentence of the type description.
• Tests: the previous "multiple values" and "text and code" failure cases become valid, plus a case for an attribute holding both a text and a date.

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

This PR relaxes validation for org.Attribute so a single attribute can contain multiple related values (e.g., reference text + reference date) under one label/key/type, enabling better round-tripping for formats like FatturaPA’s AltriDatiGestionali.

Changes:

• Update GOBL-ORG-ATTRIBUTE-02 from “exactly one value” to “at least one value” across Go validation, rule JSON, and type comments.
• Adjust tests so multi-value attributes are now valid and add coverage for a “text + date” (INTENTO-style) attribute.
• Document the behavior change in CHANGELOG.md.

Reviewed changes

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

File	Description
org/attribute.go	Relaxes attribute value validation and updates inline documentation to reflect multi-value support.
org/attribute_test.go	Updates validation tests to accept multiple value fields and adds a representative “text + date” case.
data/rules/org.json	Regenerates rule GOBL-ORG-ATTRIBUTE-02 description and expression to match the new >= 1 requirement.
CHANGELOG.md	Notes the behavioral change for org.Attribute in the Unreleased “Changed” section.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.92%. Comparing base (ee9c5a1) to head (b938329).

Additional details and impacted files

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  Coverage   94.92%   94.92%           
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  Files         325      325           
  Lines       17678    17678           
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  Hits        16780    16780           
  Misses        556      556           
  Partials      342      342           

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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.

[rilla]

Nice!