Validate media type regex for OCI artifact types per RFC 6838

[Copilot]

Verified that the regex on line 60 of ArtifactType.java correctly validates both standard media types and OCI artifact types according to RFC 6838/RFC 2045.

Validation Performed

• Confirmed regex implements RFC 2045 token specification: [A-Za-z0-9!#$%&'*+\-.^_{|}~]`
• Verified all OCI media types in codebase pass validation (e.g., application/vnd.oci.image.manifest.v1+json)
• Tested edge cases including all valid token special characters and invalid tspecials
• All unit tests pass (3/3 in ArtifactTypeTest)

Key Points

OCI artifact types are media types—both follow RFC 6838. The regex correctly:

• Validates type/subtype format with RFC 2045 tokens
• Allows valid characters: alphanumerics + ! # $ % & ' * + - . ^ _ { | } ~`
• Rejects invalid characters: spaces, control chars, tspecials (<>()@,;:\"[]?=)

No code changes required—the existing implementation is correct.

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