Add Bizkaia activity extension to TicketBAI add-on

[cavalle]

• Adds extension for the Bizcaia's activity code required to individual (not business) suppliers.

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.39%. Comparing base (4a0a25f) to head (ba5f50c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #829   +/-   ##
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  Coverage   93.39%   93.39%           
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  Files         369      369           
  Lines       20115    20121    +6     
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+ Hits        18786    18792    +6     
  Misses        888      888           
  Partials      441      441           

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[Copilot]

Pull request overview

This PR adds support for a Bizkaia-specific TicketBAI supplier extension (es-tbai-bi-activity) and enforces it via invoice validation rules for Bizkaia individual suppliers, with accompanying tests and documentation/data updates.

Changes:

• Added new TicketBAI extension definition for Bizkaia individual activity code (epígrafe), including sources and a ^\d{1,7}$ pattern.
• Added invoice validation rules: require the extension for Bizkaia individuals and validate the code format when present.
• Refactored TicketBAI region codes to exported constants and updated normalization/tests accordingly.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
data/rules/es-tbai.json	Adds rule definitions for requiring/validating the Bizkaia activity extension.
data/addons/es-tbai-v1.json	Documents the new es-tbai-bi-activity extension (name/desc/sources/pattern).
CHANGELOG.md	Adds an Unreleased changelog entry for the new extension.
addons/es/tbai/extensions.go	Defines the new extension key + adds the extension definition; factors region codes into constants.
addons/es/tbai/bill.go	Implements validation rules and guard logic for Bizkaia individual suppliers; uses region constants in normalization.
addons/es/tbai/bill_test.go	Updates region normalization assertions and adds validation test coverage for the new rules.

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[mintlify]

Docs PR opened: invopop/gobl.docs#46

Documented the new es-tbai-bi-activity extension and its two validation rules in the TicketBAI add-on page.

[mintlify]

Docs PR opened: invopop/docs.invopop.com#364

Documented the new es-tbai-bi-activity extension in the TicketBAI invoicing guide and added a changelog entry.

[samlown]

👍