WA-CI-005: Fix MountPoint.unwrap_app — next CI broken (PR #739 regression)

Regression from PR #739 (WA-RAILS7-013). `MountPoint.unwrap_app` over-traverses through engine classes (which respond to `.app`) and returns nil instead of the engine class. Fix: add `return app if app.is_a?(Class)` guard. See PR for full root cause analysis.