fix: Improve error messages for missing build artifacts

[cfsmp3]

Summary

When a test fails due to missing build artifacts, users previously saw only a generic "An error occurred" message with no details. This made it difficult to understand what went wrong.

Example: Test #7139 failed because the Windows build was still in progress (Windows builds take ~40 minutes, but the test tried to fetch the artifact immediately).

Changes

• Add error_message property to Test model to expose the last error message
• Update test detail template to show error message prominently in an alert box
• Add _diagnose_missing_artifact() helper that checks GitHub workflow status and provides specific error messages:

Scenario	Error Message
Build in progress	"Build still in progress: 'Build CCExtractor on Windows' is in_progress. Please wait for the build to complete and retry."
Build failed	"Build failed: '...' finished with conclusion 'failure'. Check the GitHub Actions logs for details."
Build succeeded but no artifact	"Artifact not found: '...' completed successfully, but no artifact was found. The artifact may have expired..."
No workflow run found	"No workflow run found: '...' has not run for commit abc1234..."

Before vs After

Before: User sees generic "An error occurred - Something went wrong while the tests were running"

After: User sees specific error like "Error: Build still in progress: 'Build CCExtractor on Windows' is in_progress. Please wait for the build to complete and retry."

Test plan

• Verify syntax is correct (Python files compile)
• Test with a commit where Windows build is in progress
• Test with a commit where build failed
• Test with a successful build to ensure no regression

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.02%. Comparing base (53b3d7f) to head (5616b9b).

Files with missing lines	Patch %	Lines
mod_ci/controllers.py	86.20%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #947      +/-   ##
==========================================
+ Coverage   86.88%   87.02%   +0.13%     
==========================================
  Files          35       35              
  Lines        3759     3791      +32     
  Branches      767      776       +9     
==========================================
+ Hits         3266     3299      +33     
+ Misses        355      354       -1     
  Partials      138      138              

Flag	Coverage Δ
unittests	87.02% <88.23%> (+0.13%)	⬆️

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud