fix(e2e): isolate kubernetes user namespace test

[drew]

Summary

Keep the Kubernetes-only user namespace e2e test out of the Docker and Podman e2e jobs by declaring it as an explicit Cargo test target gated by e2e-kubernetes.

The earlier .github/workflows/e2e-gate.yml changes were reverted; the net PR diff now only changes e2e/rust/Cargo.toml.

Related Issue

N/A

Changes

• Add an e2e-kubernetes feature for Kubernetes-specific Rust e2e tests.
• Add an explicit user_namespaces Rust e2e test target requiring e2e-kubernetes.
• Restore .github/workflows/e2e-gate.yml to match main.

Testing

• cargo test --manifest-path e2e/rust/Cargo.toml --features e2e -- --list user_namespaces
• cargo test --manifest-path e2e/rust/Cargo.toml --features e2e-docker -- --list user_namespaces
• cargo test --manifest-path e2e/rust/Cargo.toml --features e2e-kubernetes --test user_namespaces -- --list
• mise run pre-commit passes
• E2E workflow rerun passes on GitHub

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

[github-actions]

Label test:e2e applied for cc7a023. Open the existing run and click Re-run all jobs to execute with the label set. The E2E Gate check on this PR will flip green automatically once the run finishes.