fix(k8s): fail execs without an exit code

[michaelatkin31]

Summary

A Kubernetes exec response marked Success without an exit code currently becomes a false zero exit, and run-script steps discard genuine nonzero codes.

• Reject successful exec responses that omit an integer exit code.
• Propagate run-script exit codes through the hook entrypoint.
• Preserve runner file-command copying before returning the script result.

Testing

• npm run build --prefix packages/k8s
• npm run test --prefix packages/k8s -- --runTestsByPath tests/exec-response-test.ts tests/exec-pod-step-test.ts tests/run-script-step-exit-code-test.ts
• npx tsc --project packages/k8s/tsconfig.json --noEmit
• npm run lint
• npm run format-check

Context

Links: Completes DDP-1522