Strip `index.ts` and `index.js` from extension display paths in both
compact and expanded views of the startup banner. This makes the list more
readable by removing redundant filename noise.
Changes:
- Add `formatExtensionDisplayPath()` helper to centralize path formatting
- Update `getCompactExtensionLabels()` to strip index files from segments
- Update expanded view formatting to use the new helper
- Add comprehensive tests for both views
Fixes#3549Fixes#3559
Add getCommandDiagnostics and getShortcutDiagnostics mocks to fix
'Cannot read properties of undefined' errors in InteractiveMode tests.
regression from 32a305cb
