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]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props huzaifaalmesbah, mukesh27.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

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