[Feature]: Implement an ARC 7 CE - Slurm integration test in a CI workflow

[aldbr]

User Story

As a contributor for the ARC module,
I want an ARC 7 CE wired to Slurm running in CI with a real submit → monitor → retrieve test,
So that changes to the ARC path are validated end-to-end against a real LRMS and no regressions slip in.

Feature Description

Stand up the arc-slurm integration-test stack (IC-ADR-002): an ARC 7 Compute Element
(REST/AREX) wired to a single-node Slurm LRMS, plus CI that builds the image, boots it, and runs a
full submit → monitor → retrieve test with the real ARC client tools
(arcsub/arcstat/arcget). ARC's arcctl test-ca mints CA + host cert at install → no grid-PKI
in CI; the client credential is written to the shared credentials/ volume.

Harmonize onto the scaffold from # per the checklist.

Definition of Done

• Image builds & boots in CI; healthcheck (curl -k the REST info URL) passes
• arcsub submits, arcstat reaches Finished, arcget retrieves, output asserted
• ARC/Slurm logs uploaded as artifacts on every run
• Harmonized onto the scaffold

Alternatives Considered

• Official ARC image — none is maintained; pinned RPMs on EL9 is the supported path (ADR §Facts).

Related Issues

Additional Context