Incorrect template part slugs when the theme folder name ends with parts

[yashjawale]

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.

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.

Both the parts with parts/ prefix & without them, fail to save changes in block editor.

Tested with current version of trunk & also present in 7.0.

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Originally reported at Gutenberg repository in ​#42679 by @mamaduka

Trac ticket: https://core.trac.wordpress.org/ticket/65580

## Use of AI Tools

AI assistance: Yes
Tool(s): OpenCode
Model(s): DeepSeek V4 Flash
Used for: Checking initial implementation, followed by manual testing & verification

Trac ticket: https://core.trac.wordpress.org/ticket/65580

## Use of AI Tools

AI assistance: Yes
Tool(s): OpenCode
Model(s): DeepSeek V4 Flash
Used for: Checking initial implementation, followed by manual testing & verification

[yashjawale]

The small change is to use strrpos instead of strpos for determing index for substring.
So we start looking for parts from the end instead.

From my testing the change appears to work correctly, showing correct number & details of template parts & allowing edits in block editor.