test: extend PHP coverage to app and preview

[erseco]

Summary

• extend the lightweight PHP test bootstrap with the minimal contracts required by Application and the preview provider
• cover application bootstrap registration and init-script wiring
• cover preview MIME matching, screenshot rendering, fallback behavior, error logging, and legacy API behavior
• include lib/AppInfo and lib/Preview in the backend Clover report
• keep the existing 90% backend coverage threshold

Scope

This is the first PR in a stacked coverage series. It intentionally does not include controllers yet.

Stack:

1. AppInfo + Preview (this PR)
2. Small controllers
3. EditorController / extracted editor logic

Validation

• CI: green
• Playground preview: green
• backend coverage with AppInfo + Preview + Service: 90.23%
• Application: 100% lines
• ElpxPreviewProvider: 81.08% lines

Current stack and merge order

1. test: extend PHP coverage to app and preview #119 — AppInfo + Preview
2. test: cover read-only PHP controllers #120 — read-only controllers
3. test: cover page and static PHP controllers #121 — page/static controllers
4. refactor: extract legacy editor file migration #122 — extract legacy editor migration
5. test: cover editor controller orchestration #123 — EditorController + editor HTML service

These PRs are intentionally based on one another for clean review diffs. Merge #119 first, then retarget #120 to main; repeat that process for each following PR before merging it.

[github-actions]

Preview this PR in the Nextcloud Playground

A fresh Nextcloud boots in your browser with this branch's exelearning app installed and enabled (log in as admin / admin). Two sample .elpx are seeded under exelearning-samples/ in Files — click one to open the viewer.

eXeLearning editor: v4.0.5 (overlaid at boot from the upstream release).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.71%. Comparing base (dc53157) to head (43a1605).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #119      +/-   ##
============================================
- Coverage     95.01%   93.71%   -1.31%     
- Complexity       48       65      +17     
============================================
  Files            11       13       +2     
  Lines           301      350      +49     
  Branches         54       54              
============================================
+ Hits            286      328      +42     
- Misses            8       15       +7     
  Partials          7        7              

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