test(ci): enable CUDA GPU execution validation in GPU CI

[elezar]

Description

Enable the CUDA GPU execution validation test in GPU CI once a published, immutable CUDA validation image reference is available.

Issue #1472 adds the GPU validation harness and CUDA execution test. That test can run locally when OPENSHELL_E2E_GPU_CUDA_WORKLOAD_IMAGE points at a suitable image, but it should visibly skip when the variable is unset. This follow-up wires the test into CI so CUDA execution is actually enforced by the GPU e2e workflow.

Context

Parent roadmap: #1444
Depends on: #1472
Related image artifacts: #1476 and PR #1484

PR #1484 defines local GPU validation image artifacts, but does not publish stable image references for CI. This issue should be completed after there is a published CUDA validation image that CI can pull by immutable tag or digest.

Proposed Scope

• Choose or publish a stable CUDA validation image reference for CI.
• Configure GPU CI to set OPENSHELL_E2E_GPU_CUDA_WORKLOAD_IMAGE to that image.
• Ensure the Docker GPU e2e workflow runs the CUDA execution validation path, not only the skip path.
• Keep local/unconfigured runs allowed to skip when the env var is unset.

Definition of Done

• A CUDA validation image is available to CI by immutable tag or digest.
• GPU CI sets OPENSHELL_E2E_GPU_CUDA_WORKLOAD_IMAGE.
• GPU CI runs the CUDA execution validation added by Define GPU validation tests for GPU-enabled drivers #1472.
• GPU CI fails if the workload image exits unsuccessfully.
• GPU CI fails if the workload image output does not contain OPENSHELL_GPU_WORKLOAD_SUCCESS.
• Documentation names the CI image reference and explains how local developers can reproduce the same validation.