Template parts can be created unintentionally with a hybrid theme

[t-hamano]

Description

Found in this comment on the core ticket: https://core.trac.wordpress.org/ticket/58827#comment:21

Classic themes that support template parts can access the template parts parts in the Site Editor. Users should only be able to edit template parts provided by the theme and should not be allowed to create new template parts. However, it is possible to unintentionally create a new template part in the template parts editor.

Step-by-step reproduction instructions

• Access the test environment (http://localhost:8889/wp-admin).
• Activate the Emptyhybryd theme. This theme has opted in to block template part support.
• Go to Appearance > Template Parts > header > editor canvas.
• You should be able to insert template parts and create new template parts.

Screenshots, screen recording, code snippet

e16d2e236870f487f86c8fbf5af7a8af.mp4

Environment info

• WordPress Version: 6.4.2
• Gutenberg Version: 17.6.0-rc.2 (It can be reproduced regardless of whether it is enabled or disabled)

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