Improve Helm/Kubernetes readiness for streamable HTTP runtime

[vincentkoc]

Details

This PR improves Kubernetes and Helm deployment readiness for the streamable HTTP server runtime:

• Harden Docker image with multi-stage build, production deps, non-root runtime user, and container HEALTHCHECK.
• Add probe-friendly HTTP endpoints: /health, /healthz, /ready, /readyz, and /ping.
• Add graceful shutdown handling for SIGTERM/SIGINT to close transport cleanly.
• Expand transport tests to cover new probe/ping endpoints.
• Document Helm probe recommendations in streamable HTTP transport docs.

Change checklist

• User facing
• Documentation updated (if needed)
• Tests added/updated (if needed)
• Breaking changes documented (if any)

Issues

• Resolves #N/A
• OPIK- N/A

Testing

• npm run check
• npm run build

Documentation

• Updated docs/streamable-http-transport.md with endpoint additions and Kubernetes probe examples.