[Subserial]

Fixes #2433

#2267 introduced preserving the manifest media type, but was overridden by the default image type of empty.Image. As a result, flattening an OCI image produced a mixed manifest, e.g.:

• Manifest mediaType: application/vnd.oci.image.manifest.v1+json (OCI)
• Config mediaType: application/vnd.docker.container.image.v1+json (Docker)
• Layer mediaType: application/vnd.docker.image.rootfs.diff.tar.gzip (Docker)

Changes

• Preserve layer media type and config media type for OCI manifests (via stream.WithMediaType)
• Expand TestFlattenImagePreservesMediaType to assert that m.Config.MediaType and m.Layers[0].MediaType match the expected media type family.
• Add TestFlattenIndexPreservesMediaType to verify that flattening an OCI index does not emit Docker artifact types on index descriptors and preserves child image media types.

Verification

go test -v ./...

[codecov-commenter]

Codecov Report

❌ Patch coverage is 52.17391% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.62%. Comparing base (79af990) to head (4702dbf).

Files with missing lines	Patch %	Lines
cmd/crane/cmd/flatten.go	52.17%	7 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2438      +/-   ##
==========================================
+ Coverage   59.48%   59.62%   +0.14%     
==========================================
  Files         167      167              
  Lines       11619    11637      +18     
==========================================
+ Hits         6911     6939      +28     
+ Misses       3836     3812      -24     
- Partials      872      886      +14     

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