test+docs: validate ModelPack compatibility in Docker Model Runner

[rishi-jat]

Adds an end-to-end test validating the ModelPack compatibility introduced in #516.

The test verifies that a ModelPack OCI artifact can be pulled and handled by Docker Model Runner.

Also adds documentation explaining how to run ModelPack artifacts using Docker Model Runner.

Related issue: modelpack/model-spec#151

[sourcery-ai]

Hey - I've left some high level feedback:

• In the pull modelpack artifact test, comparing pulledContent and originalContent via string(...) != string(...) is unnecessary for byte slices; using bytes.Equal(pulledContent, originalContent) avoids extra allocations and handles arbitrary binary content more idiomatically.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In the `pull modelpack artifact` test, comparing `pulledContent` and `originalContent` via `string(...) != string(...)` is unnecessary for byte slices; using `bytes.Equal(pulledContent, originalContent)` avoids extra allocations and handles arbitrary binary content more idiomatically.

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[gemini-code-assist]

Code Review

This pull request adds documentation and an end-to-end test for ModelPack compatibility. The documentation changes are clear and helpful. The new test case is a good addition, but it uses a workaround that makes it not fully representative of a real-world scenario. I've added a comment with a suggestion to improve the test's accuracy, which will require a corresponding change in the validation logic.

[Copilot]

Pull request overview

Adds documentation and an end-to-end distribution test intended to validate pulling/running CNCF ModelPack OCI artifacts with Docker Model Runner.

Changes:

• Adds a new TestClientPullModel subtest that pushes and pulls a “ModelPack” artifact via the test registry and validates the pulled weights/config.
• Introduces a test-only modelPackTestArtifact helper to construct an OCI artifact with ModelPack-style layers/config.
• Adds a README section describing ModelPack media type compatibility and how to pull/run ModelPack artifacts.

Reviewed changes

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

File	Description
pkg/distribution/distribution/client_test.go	Adds an E2E pull test and helper artifact implementation for ModelPack-like images.
README.md	Documents ModelPack compatibility and expected media types for config/weight layers.

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[rishi-jat]

/cc @bergwolf
/cc @gorkem
/cc @aftersnow
/cc @chlins