Add container repository content-consistency preflight validation

Problem

Container images form a content graph: manifests reference configuration and layer blobs, while manifest lists/image indexes reference platform-specific manifests.

When repository content is synchronized, uploaded, copied, or assembled into a repository version, inconsistencies in this graph may not become visible until a client attempts to pull the affected image.

It would be useful for Pulp Container to provide a container-specific preflight validation that detects incomplete or inconsistent image content before it is published.

Proposed solution

Add an optional validation step that checks the integrity of container content before publication.

The validation should identify:

• manifests referencing missing blobs
• image indexes referencing missing manifests
• manifests whose referenced configuration is unavailable
• incomplete multi-platform image indexes
• references to content that is not present in the repository version being published

Instead of returning only the first failure, the validation should collect the affected image references and missing content so an administrator can correct the repository in a single operation.

Example:

Container repository validation failed

image: example/app:latest

Missing:
  sha256:abc... (config)
  sha256:def... (layer)

image: example/app:arm64

Missing:
  sha256:123... (manifest)

Requirements

• Validation must operate against a specific immutable repository version.
• Validation must not modify repository content.
• Validation should avoid repeatedly querying the same content objects.
• The operation should provide actionable errors suitable for both API clients and task logs.
• Validation should support multi-platform image indexes.
• Large repositories should be handled without loading the complete content graph into memory.

Acceptance criteria

• A repository version containing a complete image graph passes validation.
• Missing manifests are detected.
• Missing configuration blobs are detected.
• Missing layer blobs are detected.
• Broken image-index references are detected.
• Multiple inconsistencies are reported together.
• Validation does not modify repository content.
• Tests cover single-platform images and multi-platform indexes.
• Tests cover large numbers of referenced blobs without excessive memory usage.

Motivation

Pulp Container is responsible for distributing OCI/Docker container content. Detecting an inconsistent content graph before publication would move failures from the consumer side of the registry to the repository-management side, where they can be diagnosed and corrected before users attempt to pull the affected images.

Start by tracing the repository-version publication path and the existing handling of image manifests, configuration blobs, layers, and multi-platform indexes. Define validation against one immutable repository version, then add coverage for complete and incomplete graphs, aggregated errors, non-modification, and large referenced-blob sets with bounded memory use.