Add GitHub native coverage comments for PRs

[Copilot]

Coverage comments were not appearing on PRs because the postprocess workflow (which uploads to Codecov) only runs for master branch pushes via workflow_run trigger.

Solution

Hybrid coverage approach:

• PRs: GitHub native coverage comment in CI pipeline
• Master: Existing Codecov upload in postprocess (unchanged)

Changes

.github/workflows/ci_pipeline.yml - Added coverage_comment_on_pr job:

• Runs only for pull_request events after all test jobs complete
• Downloads coverage artifacts from test matrix jobs
• Combines coverage and posts comment via py-cov-action/python-coverage-comment-action@v3
• Uses built-in github.token, no external service dependency

coverage_comment_on_pr:
  if: github.event_name == 'pull_request' && !cancelled()
  needs: [user_cpu_tests_linux, project_cpu_tests_linux, ...]
  permissions:
    pull-requests: write

Benefits:

• Immediate PR coverage feedback without external service
• Codecov retained for master branch historical tracking and badge generation
• Simpler token management

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[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.