An incorrect template part slugs when the theme folder name ends with parts

[Mamaduka]

Description

I noticed while working on #37943.

The _get_block_templates_files generates template name slugs that include directory names when the theme name ends with parts. As a result, it causes REST API requests to fail and prevents saving the changes.

I think the following lines are the cause of the issue - https://github.com/WordPress/wordpress-develop/blob/6.0/src/wp-includes/block-template-utils.php#L316-L322.

Step-by-step reproduction instructions

1. Create a child theme using WP-CLI

wp scaffold child-theme auto-parts --parent_theme=twentytwentytwo --theme_name="Auto Parts"

2. Copy the parts directory from the parent theme.

cp -r wp-content/themes/twentytwentytwo/parts wp-content/themes/auto-parts/parts

3. Go to the Site Editor -> Template Parts
4. Confirm that template part slugs contain the directory name.

Screenshots, screen recording, code snippet

Environment info

WP 6.0.x with or without the Gutenberg plugin.

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes