fix(image): merge OS data from cached layers

[1solomonwakhungu]

Description

Image config analysis previously merged OS data only from layers analyzed during the current scan. If a layer was already cached, its OS data was omitted; when several layers were analyzed in parallel, completion order could also affect the merged result.

This change:

• reads OS information from cached layers whenever the image config needs analysis
• records parallel layer results by cache key and merges them in image layer order
• adds a small cache RPC so client/server scans can retrieve only the cached OS record
• falls back to analyzing a layer if cached OS data cannot be retrieved, preserving compatibility with older cache servers

Related issues

• Close bug: during inspectConfig, the OS information obtained from the layers is non-deterministic or missed #9982

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Testing

• GOEXPERIMENT=jsonv2 go test ./...
• go tool mage test:unit
• GOEXPERIMENT=jsonv2 go test -race ./pkg/fanal/artifact/image -run TestArtifact_InspectUsesCachedOSInLayerOrder
• GOEXPERIMENT=jsonv2 golangci-lint run --build-tags=integration --new-from-rev=upstream/main
• go tool mage protoc:breaking

The regression test forces cached layer lookups to finish out of order and verifies that config analysis still receives the OS produced by image layer order.

Checklist

• I have read the guidelines for contributing to this repository.
• I have followed the conventions in the PR title.
• I have added tests that prove the fix is effective.
• Documentation is not required because this changes internal cache and image-analysis behavior only.
• Usage information is not required because this introduces no user-facing options.
• A before/after UI example is not required because this is not a user interface change.

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