Ensure DOI can be minted when only non-SEEK creators are present

[fbacall]

Fixes #2300 for real this time

[Copilot]

Pull Request Overview

This PR fixes a bug where DOI minting was incorrectly blocked when assets had non-SEEK creators (unregistered creators via assets_creators) but no registered SEEK creators. The validation now properly accepts either type of creator.

• Updated validation logic in mint_doi to check assets_creators for models that support it, falling back to creators otherwise
• Added test coverage to verify DOI minting succeeds with non-SEEK creators present
• Enhanced existing test to verify both creators and assets_creators are empty in the error case

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/seek/doi/acts_as_doi_mintable.rb	Modified creator validation to check assets_creators if available, matching the pattern already used in datacite_metadata
test/integration/doi_minting_test.rb	Added test for non-SEEK creator DOI minting and assertion to verify both creator types in existing test

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