Ticketbai: Customer validation now only required for non-simplified invoices

[pmenendz]

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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 93.05%. Comparing base (54ba861) to head (7fdad57).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #754   +/-   ##
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  Coverage   93.05%   93.05%           
=======================================
  Files         333      333           
  Lines       17794    17798    +4     
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+ Hits        16558    16562    +4     
  Misses        870      870           
  Partials      366      366           

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[Copilot]

Pull request overview

Updates the Spain TicketBAI (es-tbai-v1) addon invoice validation rules so that customer presence and TaxID validation are enforced only for non-simplified invoices, aligning validation behavior with simplified-invoice expectations.

Changes:

• Make inv.Customer required only when the invoice is not tagged tax.TagSimplified, and only then run TicketBAI-specific customer TaxID validation.
• Add test cases covering missing customer for standard invoices, and allowing missing customer / missing customer TaxID for simplified invoices.
• Add an Unreleased changelog entry documenting the behavior change.

Reviewed changes

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

File	Description
addons/es/tbai/bill.go	Gates customer Required + TicketBAI customer validation behind !inv.HasTags(tax.TagSimplified)
addons/es/tbai/bill_test.go	Adds regression/behavior tests for standard vs simplified invoice customer requirements
CHANGELOG.md	Documents the validation behavior change under Unreleased/Changed

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