fix: re-apply JSON escaping in junit-summary custom_data (#837)

Commit 4f0b0a0 ("restore workflow consolidation reverted by PR #788")
re-created unit-tests.yml but lost the JSON escaping fix from 56a196e.
The unescaped curly braces and quotes in the custom_data parameter break
the kubeflow/pipelines junit-summary action's bash conditional, causing
the "Mark Workflow failure if test reporting failed" step to fail even
when all tests pass.

Signed-off-by: Martin Prpič <mprpic@redhat.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: mprpic

.github/workflows/unit-tests.yml

@@ -147,7 +147,7 @@ jobs:
         if: (!cancelled()) && steps.upload.outcome != 'failure'
         with:
           xml_files: '${{ env.TESTS_DIR }}/reports'
-          custom_data: '{"HTML Report": "${{ steps.upload.outputs.artifact-url }}"}'
+          custom_data: '{\"HTML Report\": \"${{ steps.upload.outputs.artifact-url }}\"}'
         continue-on-error: true
 
       - name: Publish Test Summary