Validate enabled built-in Slurm scripts

[fabrizio2210]

Summary

• verify that every enabled slurmScripts.builtIn entry has bundled script content when customContent is not provided
• verify that every enabled built-in has a bundled JSON check specification when customConfig is not provided
• fail Helm rendering with an actionable migration error when either asset is missing
• add Helm coverage for both missing-script and missing-configuration cases

Why

Helm values can retain an enabled built-in entry after that check has been removed from a newer chart version. The template previously accepted the unknown key, rendered an empty script, and attempted to parse an empty check specification. The resulting malformed checks.json could cause check_runner.py to reject the complete configuration and run no health checks.

This issue was identified while preparing #2778. It is implemented separately from main so #2778 can remain focused on removing the obsolete alloc_mem_used check.

Impact

Upgrades with stale or misspelled enabled built-in keys now fail during Helm rendering with instructions to remove the obsolete entry or provide the corresponding custom content/configuration.

Valid bundled checks and built-ins with explicit customContent or customConfig continue to render unchanged.

Validation

• helm unittest -f 'tests/slurm_scripts_test.yaml' helm/slurm-cluster — 2 tests passed
• helm unittest helm/slurm-cluster — 101 tests passed across 18 suites
• default helm template rendering passed
• git diff --check passed