Validate advances are non-negative pt-saft-v1

[cavalle]

• Adds rule to check that advance payment amounts on invoices are no less than 0 as required by the SAF-T spec.

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 a Portugal SAF-T (pt-saft-v1) validation rule to ensure invoice advance payment amounts are non-negative, aligning invoice validation behavior with the SAF-T specification.

Changes:

• Added a pt-saft invoice rule to validate that each advance payment amount is >= 0.
• Updated the generated rules data (data/rules/pt-saft.json) to include the new assertion.
• Added a unit test covering the negative-advance validation failure case.

Reviewed changes

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

File	Description
data/rules/pt-saft.json	Adds rule metadata for validating non-negative advance amounts on invoices.
addons/pt/saft/invoices.go	Enforces num.ZeroOrPositive on payment.advances[].amount.
addons/pt/saft/invoices_test.go	Adds test ensuring negative advance amounts fail validation.
CHANGELOG.md	Documents the new pt-saft-v1 validation rule in Unreleased.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.39%. Comparing base (4cdd472) to head (8d6df9b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #830   +/-   ##
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  Coverage   93.39%   93.39%           
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  Files         369      369           
  Lines       20121    20124    +3     
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+ Hits        18792    18795    +3     
  Misses        888      888           
  Partials      441      441           

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[mintlify]

Docs PR opened: invopop/gobl.docs#49

Added a validation rule entry documenting that advance payment amounts on Portuguese SAF-T invoices must not be negative.

[mintlify]

Docs PR opened: invopop/docs.invopop.com#385

Added a Portugal entry to the May 12, 2026 changelog noting the new SAF-T validation rejecting negative advance payment amounts.