What?

Change the Template Part block's render method (render_block_core_template_part) to use get_block_file_template rather than a number of lower-level functions.

Why?

Higher-level functions such as get_block_file_template apply filters (such as the eponymous get_block_file_template) that otherwise aren't invoked. (This is relevant e.g. for #51449.)

Furthermore, it can be argued that if a high-level function is available to what otherwise requires calling a number of lower-level functions, it's preferable to use the high-level function (unless there are side effects and/or performance issues, of course) 😄

How?

By using get_block_file_template (which returns a WP_Block_Template object) in the code branch that loads a block template from a theme file, thus supplanting calls to the lower-level _get_block_template_file and _inject_theme_attribute_in_block_template_content functions.

Note that get_block_file_template invokes both of them in its function body. (_inject_theme_attribute_in_block_template_content is called via _build_block_template_result_from_file).

Testing Instructions

Verify that Template Parts still work as before:

• On the frontend, with an unmodified block theme;
• In the editor -- loading, modifying, and saving;
• On the frontend, with modifications to the template part.