fix(demo): enable dummy job executor

[anpicci]

Summary

• enable the dummy job executor monitor in the standard demo deployment;
• set the validated task settings explicitly through diracx.settings;
• keep production/default scheduling disabled in the corresponding DIRACX implementation.

Coordination

This is the charts-side companion to DIRACGrid/diracx#995 and specifically
completes the run_demo part of its review feedback.

The corresponding DIRACX implementation is commit
c9d9a31d1e27bd5831db0ecfa96f8f12cc61d34d.

Validation

• rendered demo/values.tpl.yaml with all placeholders resolved;
• helm lint passed;
• helm template passed;
• structured YAML assertions verified both settings in diracx-secrets;
• structured YAML assertions verified the task scheduler imports diracx-secrets;
• non-Docker pre-commit hooks passed;
• Docker-backed pre-commit hooks were unavailable locally because Docker Desktop was not running, so CI should provide the final full-hook validation.