fix: stop changing runtime error settings and trim unused core includes

[erseco]

Stacked on #148, which stops code-only changes from rewriting the translation catalogs. This PR therefore touches no files under languages/.

WordPress.org review findings addressed:

Runtime error settings

The standalone editor page called ini_set( 'display_errors', '0' ) twice and error_reporting( 0 ), which changed the site's PHP configuration for that request. The page already starts an output buffer, and send_and_exit() discards everything printed before the editor document, so the buffer alone keeps the first byte clean. The three calls and their phpcs:ignores are removed.

EditorPageTest now asserts that error_reporting() and display_errors are unchanged after the editor boots. Before, the test restored them.

Core admin includes

• REST_API::create_elp_file() loaded wp-admin/includes/media.php and image.php but used no function from either. Both are removed.
• wp-admin/includes/file.php stays in both REST handlers (/create and /save/{id}), because wp_handle_upload() is not loaded in REST requests. Each require_once now sits immediately before its wp_handle_upload() call.
• The admin-post upload handler (admin/class-admin-upload.php) that the review also cited was removed as dead code in Remove unreachable scaffolding and clean options on uninstall #142.
• Nothing else in the plugin loads a core admin file. The remaining admin functions (get_current_screen(), submit_button(), add_meta_box()) only run on admin screens where core has already loaded them.

Verification

PHPUnit: OK (911 tests). PHPCS clean. No error_reporting, display_errors or ini_set remains in plugin PHP.

[github-actions]

Test in WordPress Playground

Test the plugin with the code from this branch:

ℹ️ The eXeLearning editor is fetched from the shared release and unpacked into the plugin when the playground boots, so the first load may take a few extra seconds. ELP upload, shortcode, Gutenberg block and preview work normally.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.40%. Comparing base (bc32141) to head (3a77417).

Additional details and impacted files

@@                        Coverage Diff                        @@
##             feature/pot-file-references     #124      +/-   ##
=================================================================
+ Coverage                          95.71%   96.40%   +0.69%     
- Complexity                             0      827     +827     
=================================================================
  Files                                  7       36      +29     
  Lines                               1049     4228    +3179     
=================================================================
+ Hits                                1004     4076    +3072     
- Misses                                45      152     +107     

Flag	Coverage Δ
javascript	95.71% <ø> (ø)
php	96.63% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
includes/class-exelearning-editor.php	93.25% <ø> (ø)
includes/class-exelearning-rest-api.php	99.70% <100.00%> (ø)

... and 27 files with indirect coverage changes

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