fix: validate mode input for end-to-end testing workflow

Author: ChristophShyper

.github/workflows/e2e-action-container-structure-test.yml

@@ -37,6 +37,16 @@ jobs:
     name: Basic test with text output format
     runs-on: ubuntu-latest
     steps:
+      - name: Validate mode input
+        run: |
+          case "${{ inputs.mode }}" in
+            ref|image) ;;
+            *)
+              echo "Unsupported mode '${{ inputs.mode }}'. Expected 'ref' or 'image'."
+              exit 1
+              ;;
+          esac
+
       - name: Checkout repository
         uses: actions/checkout@v6