Media: Assign the HEIC upload error flag to the Plupload settings

[itzmekhokan]

wp_show_heic_upload_error() assigned the heic_upload_error flag to an undeclared $plupload_init variable instead of the $plupload_settings parameter it receives, so the callback returned the settings untouched and the flag was never added.

What the problem was:

• The typo was introduced in [58849] (#53645, shipped in 6.7.0); before that changeset the function assigned to $plupload_settings correctly.
• The plupload_default_settings path is unaffected in practice, because wp_plupload_default_settings() already sets the flag inline.
• The plupload_init path is broken: media_upload_form() sets the WebP and AVIF flags inline but relies on this callback for HEIC, so no error was shown on media-new.php or in the media-upload.php iframe when the server could not edit HEIC images.

What the fix does:

• Assigns to $plupload_settings, restoring the behaviour the function has documented since 5.5.0.
• Adds a regression test for the callback.

Approach and why:

• One-character-scope fix on the assignment target — nothing else in the function or its two callers needs to change.
• The heic_upload_error flag is a non-blocking notice in handlers.js; unlike WebP and AVIF it does not remove the file from the queue, so restoring it warns the user without preventing the upload.
• The redundant inline HEIC check in wp_plupload_default_settings() was deliberately left alone: it is harmless (the callback assigns the same true) and removing it would change behaviour for anyone who has unhooked the callback.

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

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