Describe the bug
I'm following the instructions to develop a theme for Full Site Editing at
https://developer.wordpress.org/block-editor/tutorials/block-based-themes/
and received the following message "Template Part Not Found" on my Windows server.

I discovered that validate_file() returns 2 for a fully qualified file e.g.
C:/apache/htdocs/wp56/wp-content/themes/fizzie/block-template-parts/header.html.

gutenberg_render_block_core_template_part() doesn't cater for this value.
So the function fails to load the template part.

To reproduce
Steps to reproduce the behavior:

1. On a Windows server
2. Follow instructions to develop a theme for Full Site Editing
3. View some content. e.g. Visit site.
4. See the Template Part Not Found as part of the output.

Expected behavior
Code should be able to load template part files in a Windows server environment.

I suggest either change the logic to support a return value of 2 from validate_file().
or don't call validate_file() and validate the value for the slug attribute.

Screenshots

If applicable, add screenshots to help explain your problem.

Editor version (please complete the following information):

• WordPress version: [e.g: 5.3.2] 5.5.1
• Does the website has Gutenberg plugin installed, or is it using the block editor that comes by default? [e.g: "gutenberg plugin", "default"] Gutenberg 5.5.1 with Full Site Editing selected.
• If the Gutenberg plugin is installed, which version is it? [e.g., 7.6]

Desktop (please complete the following information):

• OS: [e.g. iOS] Windows
• Browser [e.g. chrome, safari] Chrome
• Version [e.g. 22]

Additional context

I've implemented a local fix

$valid = validate_file( $template_part_file_path );
if ( ( 0 === $valid || 2 == $valid )  && file_exists( $template_part_file_path ) ) {
	$content = file_get_contents( $template_part_file_path );
}