Add Helm chart deployment test

SeaTunnel's Helm CI validated chart syntax and rendering, but it did not install the chart. A chart could pass lint and template checks while still failing to deploy because of image, readiness, selector, RBAC, or runtime registration problems.

Added a Helm test script with render and install modes, pinned and checksum-verified Helm setup, change-gated K3s installation, rollout checks for master and worker deployments, REST polling for cluster member registration, and failure diagnostics for pods and logs.

• Raises confidence that SeaTunnel Helm chart changes produce a deployable Kubernetes installation, not just valid YAML.
• Keeps expensive K3s installation scoped to chart changes while preserving strict lint and template validation for backend checks.
• Improves CI diagnostics by printing Kubernetes resources, pod descriptions, and logs when installation fails.

• Added `deploy/kubernetes/helm-tests/test-chart.sh` with render and install paths for Helm chart validation.
• Verified local shell syntax and render mode, including strict lint and default plus ingress/TLS template rendering.
• PR review evidence confirms the CI install path ran on K3s and verified cluster membership through the real REST endpoint.