CI: Fix "Publish Test Results" job to actually fail when tests fail

[Copilot]

Summary

The EnricoMi/publish-unit-test-result-action with fail_on: test failures only marks the "NUnit Tests" check run (created via the Checks API) as failed — it does not fail the GitHub Actions job step. The "Publish Test Results" job therefore always concluded success, making it useless as a branch protection guard. PRs with failing tests could be merged unimpeded.

Two changes to CI.yml:

• comment_mode conditional — prevents the publish step from posting/updating comments on already-closed PRs when CI runs on push to main (previously always, now off for non-PR events)
• Explicit failure propagation — adds a final step that exits 1 if debug_build_and_test did not succeed, so the "Publish Test Results" job actually fails and branch protection blocks the merge:

- name: Fail if build or tests failed
  if: ${{ needs.debug_build_and_test.result != 'success' }}
  run: |
    echo "::error::Build or test failures detected in 'Build Debug and run managed tests' job"
    exit 1

No branch protection rule changes needed — the existing "Publish Test Results" requirement now works correctly.

CI-ready checklist

• Commit messages follow .github/commit-guidelines.md (subject ≤ 72 chars, no trailing punctuation; if body present, blank line then ≤ 80-char lines).
• No whitespace warnings locally:

  git fetch origin
  git log --check --pretty=format:"---% h% s" origin/<base>..
  git diff --check --cached

• Builds/tests pass locally (or I've run the CI-style build via Bash script or MSBuild).
• For any Src/** folders touched, corresponding AGENTS.md files are updated or explicitly confirmed still accurate.

Notes for reviewers (optional)

The fail_on: test failures setting is kept — it still correctly marks the "NUnit Tests" check run as failed, which surfaces individual test failure annotations. The new explicit step is additive: it propagates the upstream job result to the "Publish Test Results" job conclusion so branch protection actually enforces passing tests.

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This change is 

[Copilot]

@jasonleenaylor Unfortunately, I hit an error while trying to use the custom Copilot setup steps configured for this repository and had to close this PR. The error I am seeing is:

no `copilot-setup-steps` job found in your `copilot-setup-steps.yml` workflow file. Please ensure you have a single job named `copilot-setup-steps`. For more details, see https://gh.io/copilot/actions-setup-steps

Once you or someone with the necessary access fixes the problem, please unassign and then reassign issue #0 to me and I'll retry. Thanks!

[Copilot]

@jasonleenaylor Unfortunately, I hit an error while trying to use the custom Copilot setup steps configured for this repository and had to close this PR. The error I am seeing is:

no `copilot-setup-steps` job found in your `copilot-setup-steps.yml` workflow file. Please ensure you have a single job named `copilot-setup-steps`. For more details, see https://gh.io/copilot/actions-setup-steps

Once you or someone with the necessary access fixes the problem, please unassign and then reassign issue #0 to me and I'll retry. Thanks!