ci: add Helm lint values matrix for multi-config coverage

[yizhaodev]

Summary

The current helm lint step in the pre-commit workflow (#411) only runs against the default values.yaml. This misses configuration variants that could break templates.

Proposed values matrix

Add a charts/batch-gateway/ci/ directory with variant values files, and run helm lint against each:

1. values-global-gateway.yaml — globalInferenceGateway mode (single gateway)
2. values-model-gateways.yaml — modelGateways mode (per-model routing)
3. values-flow-control.yaml — flow control enabled with inferenceObjective
4. values-minimal.yaml — only required fields set

Update the CI step to lint all variants:

- name: Lint Helm chart
  run: |
    helm lint charts/batch-gateway
    for f in charts/batch-gateway/ci/values-*.yaml; do
      helm lint charts/batch-gateway -f "$f"
    done

Related

• ci: add helm lint and unittest to PR workflow #411 (added initial helm lint to CI)