Add matrix schema validation to shared local/GHA CI path

[mike-dubman]

Goal

Add schema validation for CI matrix YAML files in the same execution path used by both local and GitHub Actions runs.

What changed

• Updated schema_validator/ci_demo_schema.yaml to align with currently supported Matrix.groovy configuration syntax.
• Added schema validation step in scripts/local_gha_ci.sh before Jenkins job execution.
• Baked validator runtime into .github/Dockerfile.jenkins (python3 + venv with yamale) so validation does not install packages at runtime.

Result

• Unknown/unsupported keys now fail fast during matrix validation.
• Local make -C .ci local-gha-ci and GHA both use the same validation flow and fail early on invalid matrix config.