Enable cross-platform integration tests on pull requests [AI-assisted] (#17)

Fixes #15

Update the cross-platform-integration job to run on pull requests
in addition to pushes to main branch. This allows catching platform-
specific issues (Windows/macOS/Linux) before merging to main.

Changes:
- Modified workflow condition from:
  if: github.event_name == 'push' && github.ref == 'refs/heads/main'
- To:
  if: |
    (github.event_name == 'push' && github.ref == 'refs/heads/main') ||
    github.event_name == 'pull_request'

Benefits:
- Catch platform-specific bugs before merging
- Higher confidence in code quality across all supported platforms
- Cleaner main branch history with fewer post-merge fixes

Tradeoffs:
- Increased CI time for PRs (~10-15 minutes)
- Higher CI resource usage per PR

Author: florath

.github/workflows/ci.yml

@@ -191,7 +191,9 @@ jobs:
     name: Cross-Platform Integration Tests
     runs-on: ${{ matrix.os }}
     needs: [lint-and-type-check, test]
-    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+    if: |
+      (github.event_name == 'push' && github.ref == 'refs/heads/main') ||
+      github.event_name == 'pull_request'
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]