Fall back to config media type for old-style artifact types

[Kunalbehbud]

What this PR does / why we need it:

Addresses Issue 1 of #2015 (the config.mediaType fallback for descriptor artifactType), following @TerryHowe's assessment on the issue.

When oras manifest index create/update enriches a child image manifest descriptor, enrichDescriptor set desc.ArtifactType = manifest.ArtifactType and left it empty for old-style artifacts — those created before the manifest artifactType field existed, which convey their type through config.mediaType instead (e.g. historically-created artifacts like OpenTofu providers).

The image-spec descriptor guidance says artifactType "is the value of the config descriptor mediaType when the descriptor references an image manifest", and the manifest guidelines explicitly call for tooling to "fallback to the config.mediaType value". This change does that: when artifactType is empty and the config is not a standard OCI image config, the enriched descriptor falls back to config.mediaType.

enrichDescriptor is shared by both the index create and update paths, so a single change fixes both (the issue assessment expected two edits).

This PR intentionally scopes to Issue 1 only. Issue 2 (a mechanism/flag to attach platform to artifact descriptors) is a larger, separate change and is left for a follow-up.

Which issue(s) this PR fixes:
Fixes #2015

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test? — Added two Test_enrichDescriptor cases: the fallback path (empty artifactType + custom config.mediaType), and the guard that keeps artifactType empty when the config is the standard OCI image config type. This closes the "no test for the old-style fallback path" gap noted in the issue.
• Does this change require a documentation update? — No; behavior now matches the documented spec expectation.
• Does this introduce breaking changes that would require an announcement or bumping the major version? — No; it only populates a descriptor field that was previously left empty for these artifacts.
• Do all new files have an appropriate license header? — No new files.

[Kunalbehbud]

Good catch — applied. Switched to the switch-based guard so the config media type is only used when it isn't a standard image config: ocispec.MediaTypeImageConfig, ocispec.MediaTypeEmptyJSON, and the Docker image config are all excluded.

docker.MediaTypeConfig didn't exist yet, so I added it to internal/docker alongside the existing manifest media types (application/vnd.docker.container.image.v1+json).

Also extended Test_enrichDescriptor to cover all three excluded config types staying empty, on top of the fallback case. PTAL.

[TerryHowe]

/lgtm

[TerryHowe]

In test/e2e/internal/testdata/nonjson_config/const.go:

var (
      Descriptor = ocispec.Descriptor{
              MediaType:    "application/vnd.oci.image.manifest.v1+json",
              Digest:       "sha256:9d16f5505246424aed7116cb21216704ba8c919997d0f1f37e154c11d509e1d2",
              Size:         529,
              ArtifactType: "application/vnd.unknown.config.v1+json", // ← add this
      }
)

[TerryHowe]

Test is broken

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.59%. Comparing base (e58fee3) to head (990e68f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2114      +/-   ##
==========================================
+ Coverage   57.52%   57.59%   +0.06%     
==========================================
  Files         139      139              
  Lines        6067     6072       +5     
==========================================
+ Hits         3490     3497       +7     
+ Misses       2357     2356       -1     
+ Partials      220      219       -1     

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