Coding Standards: Rename variables for WP 7.0

[huzaifaalmesbah]

This PR updates variable names in wp-admin includes to strictly adhere to WordPress Coding Standards (lowercase with underscores, no unnecessary abbreviations). These changes focus on local variables to avoid backward compatibility issues.

Trac ticket: https://core.trac.wordpress.org/ticket/64226

Changes

• src/wp-admin/includes/post.php: Renamed $id3data to $id3_data in edit_post().
• src/wp-admin/includes/media.php: Renamed $ext to $extension and $ext_type to $extension_type in media_handle_upload() and media_send_to_editor().
• src/wp-admin/includes/image-edit.php: Renamed $ext to $extension in wp_image_editor().

Testing Instructions

1. Media Upload: Upload image, audio, and video files. Ensure uploads complete successfully and correct icons/metadata are displayed.
2. Image Editing: Go to Media Library, select an image, click "Edit Image", perform a crop or rotation, and save. Confirm the edit is applied successfully.
3. General Admin: Edit a post and verify the page loads and saves correctly without any PHP notices or errors.

[github-actions]

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Props huzaifaalmesbah.

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[github-actions]

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