feat(gen-owl): emit DCMI agent metadata as IRIs

[jdsika]

Summary

Stacked on #33 (base feat/owlgen-iri-valued-metadata); the diff is the delta only. Refs linkml#3546.

DCMI Metadata Terms give dcterms:creator, dcterms:contributor, dcterms:publisher and dcterms:rightsHolder the range dcterms:Agent: their value is a resource, like the properties #33 emits as IRIs. As annotations they still came out as literals, so an ontology header could name its publisher only as "https://..." text.

They join IRI_VALUED_METADATA_PROPERTIES. As for every property in it, a value that is an absolute IRI or a CURIE with a declared prefix becomes an IRI, and an agent named in plain text keeps its literal:

annotations:
  dcterms:creator: The Example Team
  dcterms:publisher: https://example.org/team

<https://example.org/onto/v2>
    dcterms:creator "The Example Team" ;
    dcterms:publisher <https://example.org/team> .

The contributors metamodel slot (dcterms:contributor, range uriorcurie) already emitted IRIs; this makes the same property behave the same way as an annotation.

How was this tested?

• test_iri_valued_metadata_is_emitted_as_iris asserts dcterms:publisher (absolute IRI) and dcterms:rightsHolder (expanded CURIE) as IRIs; both assertions fail without the change.
• test_literal_valued_metadata_stays_literal keeps dcterms:creator: The Example Team as a literal.
• pytest tests/linkml/test_generators -m "not kroki": 1776 passed, 41 skipped, 3 xfailed.
• pytest tests/linkml/test_base/test_metamodel.py tests/linkml/test_biolink_model --with-slow --with-biolink: 21 passed, 2 skipped; meta.owl and biolink.owl.ttl unchanged (neither carries DCMI agent annotations).
• ruff check and ruff format --check (0.11.13) clean.

Areas of uncertainty

• Other DCMI properties have non-literal ranges too (dcterms:rights, dcterms:language, dcterms:audience, …). They are left out: their values are commonly written as text, and the list stays limited to properties whose values are resources that are linked to.

Checklist

• My code follows the contributor guidelines
• I have added tests that prove my fix/feature works
• Existing tests pass locally with my changes

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