Refactor: move image reference resolution into setup-test-environment action

[guanzhousongmicrosoft]

Context

After PR #275 merges (removing K8s < 1.35 combined-mode fallback), the "Determine DocumentDB and Gateway image references" step in test-E2E.yml (lines 147-163) can be simplified and moved into the .github/actions/setup-test-environment composite action.

Problem

The image reference construction logic is duplicated or could be duplicated across multiple test workflows (test-E2E.yml, test-integration.yml, test-backup-and-restore.yml, test-upgrade-and-rollback.yml). Each workflow independently determines DOCUMENTDB_IMAGE_REF and GATEWAY_IMAGE_REF before passing them to setup-test-environment.

Proposed Change

1. Move the image reference resolution logic into .github/actions/setup-test-environment/action.yml
2. The action should accept the raw inputs (image tag, architecture, repository owner) and derive the full image references internally
3. Remove the separate "Determine DocumentDB and Gateway image references" step from each workflow

Depends On

• PR Remove Kubernetes < 1.35 fallback, require ImageVolume (K8s 1.35+) #275 (Remove K8s < 1.35 fallback) must be merged first, as it removes the combined-mode branch from this logic

Files to Change

• .github/actions/setup-test-environment/action.yml
• .github/workflows/test-E2E.yml
• .github/workflows/test-integration.yml (if applicable)
• .github/workflows/test-backup-and-restore.yml (if applicable)
• .github/workflows/test-upgrade-and-rollback.yml (if applicable)