fix(image-spec): Several fixes in image-spec

[craciunoiuc]

No description provided.

[Copilot]

Pull request overview

This PR fixes two correctness issues in the image-spec package's tarball export path. First, it avoids a potential nil pointer dereference in SaveContent when an image has no Image config. Second, and more importantly, it fixes a corrupt-tarball bug: SaveContent always returns an image index descriptor, and handing that index directly to archive.Export (with no platform option) caused the exporter to write the index blob into index.json as a manifest entry with the application/vnd.oci.image.index.v1+json media type, which registries reject. The fix resolves the index to its child manifest descriptors and exports those. A new test verifies the resulting archive's index.json references only manifests, and go.work.sum is updated with new dependency checksums.

Changes:

• Guard image.Image with a nil check before dereferencing Platform in ctrd_save.go.
• Add manifestDescriptors in backend_archive.go to expand an index descriptor into its child manifests and export each via archive.WithManifest.
• Add TestSaveTarballIndexReferencesManifests validating archive index.json contents, plus go.work.sum checksum updates.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
image-spec/ctrd_save.go	Adds a nil check before dereferencing image.Image.Platform, preventing a panic.
image-spec/backend_archive.go	Resolves the index descriptor to child manifest descriptors before export to avoid emitting an index-typed entry in index.json.
image-spec/backend_test.go	Adds a test asserting exported index.json references only manifest-typed, well-formed manifest blobs.
go.work.sum	Auto-generated dependency checksum additions/removals.

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