Media: Fix wp_show_heic_upload_error() assigning to an undeclared variable.

[softglazee]

The wp_show_heic_upload_error() function assigned the HEIC upload error flag to an undeclared $plupload_init variable instead of the $plupload_settings parameter. As a result, the flag never reached the returned array. This behavior was introduced in [58849] and shipped in WordPress 6.7.0.

This patch updates the variable name to $plupload_settings so the flag is properly appended, and adds unit tests covering both the supported and unsupported HEIC editor cases.

Testing Instructions

These tests were run on trunk 8c3c976c25 using PHP 8.3.32 and PHPUnit 9.6.35.

1. Run the newly added tests:
   npm run test:php -- --filter test_wp_show_heic_upload_error
2. Confirm the tests pass with the fix:
   OK (2 tests, 7 assertions)
3. Revert the change in src/wp-includes/media.php to test the baseline failure:
   git stash push src/wp-includes/media.php
4. Re-run the tests:
   npm run test:php -- --filter test_wp_show_heic_upload_error
5. Confirm the expected failure occurs without the patch:
   1) Tests_Media::test_wp_show_heic_upload_error_adds_flag_when_not_supported
Failed asserting that an array has the key 'heic_upload_error'.
6. Restore the change:
   git stash pop

Trac ticket: https://core.trac.wordpress.org/ticket/65802

Use of AI Tools

Used for: Investigating the underlying code paths, structuring the PHPUnit test scaffolding, and generating shell commands for the environment. The initial finding, manual measurements, Trac ticket description, and commit message are my own work.

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