pl-favat-v3: preceding no longer required when it is a credit note

[pmenendz]

• It is not required

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

Only after checking off all the previous items:

• Marked this PR as ready for review and requested one from @samlown.

[Copilot]

Pull request overview

This PR removes the requirement for preceding invoices in credit notes for the Polish FA VAT v3 addon, making them optional instead of mandatory. This aligns with Polish KSeF FA_VAT requirements where credit notes can be issued without referencing a specific preceding invoice.

Changes:

• Removed the validation requirement that credit notes must have preceding invoices in the Polish FA VAT v3 addon
• Updated tests to verify credit notes are valid without preceding invoices
• Documented the change in the CHANGELOG

Reviewed changes

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

File	Description
addons/pl/favat/bill.go	Removed the validation.Required constraint on inv.Preceding for credit notes, making preceding invoices optional
addons/pl/favat/bill_test.go	Updated test to expect successful validation for credit notes without preceding invoices
CHANGELOG.md	Added entry documenting the removal of the preceding requirement for credit notes

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.86%. Comparing base (8ca0f6c) to head (6d8748f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #718      +/-   ##
==========================================
- Coverage   92.86%   92.86%   -0.01%     
==========================================
  Files         331      331              
  Lines       17260    17256       -4     
==========================================
- Hits        16029    16025       -4     
  Misses        867      867              
  Partials      364      364              

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