Templates Controller: Fix slug pattern to allow 404 templates

[ockham]

Backport of WordPress/gutenberg#37579. Props @PatelUtkarsh.

Quoting WordPress/gutenberg#37579:

Description

Adding 404 template from /wp-admin/site-editor.php?postType=wp_template causes 400 with rest_invalid_pattern

How has this been tested?

• Theme without 404 templates.
• Open /wp-admin/site-editor.php?postType=wp_template
• Add new 404 templates.
• Used nightly WordPress build to test this.

Screenshots

Bug:

adding-404-template.mp4

Trac ticket: https://core.trac.wordpress.org/ticket/54680

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[ockham]

cc/ @hellofromtonya @noisysocks @youknowriad -- I think this should go into 5.9 :)

[SergeyBiryukov]

Thanks for the PR! This looks good to me. It would also be great to include a unit test for this, some other tests in the phpunit/tests/rest-api/wpRestTemplatesController.php file could be used as a reference.

[ockham]

Thanks for the PR! This looks good to me. It would also be great to include a unit test for this, some other tests in the phpunit/tests/rest-api/wpRestTemplatesController.php file could be used as a reference.

Sure, happy to add a test!

[hellofromtonya]

Thanks for the integration test @ockham! LGTM 👍 Ready for commit.

[ockham]

Thanks a lot for reviewing @SergeyBiryukov @rachelbaker @hellofromtonya!

Since I don't have Core commit privileges myself, can I ask one of you kind souls to commit this? 😄

[audrasjb]

Committed in https://core.trac.wordpress.org/changeset/52436