test: expand BuildCollectionArtifact test coverage

[pavelanni]

From code review of PR #15.

TestBuildCollectionArtifact only asserts that the digest is non-empty. It should also verify:

• Manifest has correct ArtifactType
• Annotations are populated (title, version, collection name)
• Layer media type is application/vnd.skillimage.collection.v1+yaml
• Round-trip via extractCollectionYAML returns the original collection

File: pkg/oci/collection_test.go