Non-error, unexpected response for _wp-find-template not handled

[strarsis]

Description

When a non-error, unexpected response (not HTTP 404, but 200) is sent for a _wp-find-template request, e.g. a HTML page,
no error or warning message is emitted, instead the Gutenberg site editor simply stays blank, without any indication for why it fails.

Related issue: #37236
The added error handling doesn't validate the response schema/data. A HTTP 400 response with HTML will not cause an error, instead the editor simply stays blank.

That a HTML response is sent back for _wp-find-template can be caused by loaders in themes that add support for additional languages, e.g. Blade-PHP for hybrid FSE themes.

Step-by-step reproduction instructions

1. Create a theme that supports Full Site Editing (FSE).
2. Simulate a bug where a non-error, unexpected error response is send for _wp-find-template requests from the Gutenberg site editor.
3. Open the Gutenberg site editor, note that it does a _wp-find-template request, receives the unexpected, non-error response and stays blank.

Screenshots, screen recording, code snippet

No response

Environment info

• WordPress version 6.0.3, Gutenberg version 14.3.1 .

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

[strarsis]

The same issue also occurs when remove_theme_support('block-templates'); is used but a theme.json and block-templates/-parts are included, with which the (FSE) site editor is offered. The Design → Editor feature is still available, but the editor then fails to initialize.