ARCHVTEAMS-1583 serialize k8s-training CI and fix kuberay test variable

[aaronbfagan]

Release Notes (Mandatory Description)

This PR introduces a minimal CI hardening change for k8s-training only:

1. Serialize k8s-training Terraform Plan/Test runs across PRs.
2. Fix a stale KubeRay test variable name in k8s-training tests.

Problem

k8s-training runs in a shared, GPU-constrained test environment. Parallel PR runs can contend for the same limited resources and produce flaky/non-deterministic failures.

Additionally, the KubeRay test suite referenced a deprecated variable name (enable_kuberay) that is no longer declared by the module, causing warnings and reducing test signal quality.

Changes

1) CI serialization for k8s-training only

Updated .github/workflows/terraform.yml terraform matrix job concurrency:

• k8s-training uses a fixed concurrency group: k8s-training-gpu-ci
• Non-k8s-training solutions keep per-run unique concurrency groups
• cancel-in-progress: false ensures queued runs wait instead of canceling active runs

This keeps the change narrowly scoped to the path with known shared-capacity contention.

2) KubeRay test variable fix

Updated k8s-training/tests/k8s-training-kuberay.tftest.hcl:

• Replaced enable_kuberay = true
• With enable_kuberay_cluster = true

This aligns the test with current module inputs and removes undeclared-variable warnings.

Why this approach

This is intentionally a small, self-contained change set to reduce risk:

• No shared module behavior changes
• No cleanup behavior changes
• No resource naming changes
• No impact to non-k8s-training test paths beyond existing run-level behavior

Expected outcome

• k8s-training Plan/Test jobs no longer execute concurrently across PRs.
• KubeRay test no longer emits undeclared-variable warning for enable_kuberay.

Validation performed

• Confirmed diff scope is limited to:
  • .github/workflows/terraform.yml
  • k8s-training/tests/k8s-training-kuberay.tftest.hcl
• Verified workflow expression targets only k8s-training for global serialization.
• Verified test variable now matches declared module variable name.

DoD alignment

• k8s-training CI execution is single-threaded across PRs.
• Test configuration reflects current module variable interface.
• Changes are minimal and isolated to avoid regressions in unrelated paths.