Template Part block: Fix template part path arg missing from actions

[ockham]

What?

In the Template Part block, set the $template_part_file_path variable (for use by the render_block_core_template_part_file and render_block_core_template_part_none filters).

Why?

Fixes #56013. This was broken by #52892.

How?

By calling _get_block_template_file() (which duplicates some logic also found deeper in the call stack of get_block_file_template). But alas, the relevant information cannot be inferred from the WP_Block_Template object returned by get_block_file_template, nor does the eponymous filter expose any useful information.

Testing Instructions

Quoting #56013:

add_action( 'render_block_core_template_part_file', function( $template_part_id, $attributes, $template_part_file_path ) {
    var_dump( $template_part_file_path );
}, 10, 3 );

Load the front page of a vanilla WordPress site using the Twenty Twenty Four theme and observe that [...] the template part file path [is logged].

This [fix] can also be [tested] by installing the Query Monitor plugin and viewing the "Template" panel, which [should show] the path to each Template Part block that was loaded on the page.

[github-actions]

Flaky tests detected in dddcea7.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/7102449719
📝 Reported issues:

• [Flaky Test] should display the "Manage Fonts" icon #56776 in /test/e2e/specs/site-editor/font-library.spec.js
• [Flaky Test] should move to the start of the first line on ArrowUp (-firefox) #56087 in /test/e2e/specs/editor/various/writing-flow.spec.js

[sirreal]

I've tested this and it's working as expected, it fixes the mentioned issue.