fix: harden integration test result reporting

[jr200]

Summary

• Stop installing yq in the reusable integration workflow; ARC runner images are expected to provide it
• Add an explicit yq --version check so runner-image drift fails clearly
• Default empty discovery outputs to count=0 and services_tested=[] for callback JSON
• Preserve failed discovery as a failed integration result instead of treating it as no services

Failing run

• https://github.com/whengas/whengas-iac/actions/runs/27310685515/job/80679871826

Verification

• yq e . .github/workflows/run_integration_tests.yaml
• kubectl exec -n arc-runners-whengas arc-linux-whengas-x4wsv-runner-8gl2x -c runner -- yq --version -> v4.53.2