fix(unikontainers): reject OCI specs with missing root section

[Utkarsh-sharma47]

Summary

New() and Get() validate that the OCI spec contains a linux section, but did not validate that root is present.

As a result, a structurally incomplete config such as {"linux":{}} could reach code that dereferences spec.Root.Path, causing a nil pointer panic. With valid unikernel annotations, New() could also return an object with Spec.Root == nil.

This PR:

• validates that both linux and root are present before further processing
• preserves the existing missing-linux validation behavior
• adds regression coverage for New() and Get()
• verifies that valid unikernel annotations cannot bypass the missing-root check

Fixes #902

Testing

go test ./pkg/unikontainers/... -count=1

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