fix: validate Linux spec early to prevent nil dereference panics

[rishi-jat]

Description

Validate the OCI Linux spec early during container initialization and retrieval to prevent nil dereference panics caused by unsafe access to Spec.Linux.

This change establishes the Linux spec invariant once in:

• New()
• Get()

and returns an explicit error when the Linux section is missing.

Additionally, Exec() now guards access to Spec.Linux.Resources.Memory since Resources remains optional even when the Linux spec is present.

Related issues

• Fixes fix: add nil checks for optional Linux spec fields to prevent panics #409

How was this tested?

gofmt -w pkg/unikontainers/unikontainers.go
go test $(go list ./... | grep -v /tests/e2e)

LLM usage

N/A

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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[rishi-jat]

/cc @cmainas

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