test: restore concurrent demo execution

[luoliwoshang]

Summary

• restore RunAllGenPkgDemos to concurrent demo execution
• collect failures via failedDemosCh and report aggregated failures

Notes

• this branch currently only carries the requested restoration commit

[gemini-code-assist]

Note

Gemini is unable to generate a summary for this pull request due to the file types involved not being currently supported.

[xgopilot]

Review Summary

This PR appears to have no code changes. The diff is empty - the PR commit (78051e7) contains no file modifications compared to its parent commit.

Observations:

• The PR description mentions restoring RunAllGenPkgDemos to concurrent demo execution
• However, the current main branch already has concurrent execution implemented via goroutines (see cmd/llcppgtest/demo/demo.go:234-244)
• The branch is 14 commits behind main and 1 commit ahead, but that commit contains no changes

Recommendation: Please verify the intended changes were committed. It appears the commit may have been created without staging the intended modifications, or the feature was already merged to main via a different PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.06%. Comparing base (2f8e292) to head (78051e7).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #662      +/-   ##
==========================================
- Coverage   84.74%   84.06%   -0.69%     
==========================================
  Files          27       28       +1     
  Lines        2314     2773     +459     
==========================================
+ Hits         1961     2331     +370     
- Misses        310      392      +82     
- Partials       43       50       +7     

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