Backport: Store legacy sidebars when switching to a block theme

[Mamaduka]

Trac ticket: https://core.trac.wordpress.org/ticket/57531
Related Gutenberg PR: WordPress/gutenberg#45509

Testing instructions without package updates

1. Switch to the Twenty Twenty theme.
2. Confirm sidebars have at least one widget by going to Appearance > Widgets
3. Switch to any block theme. I was using Twenty Twenty-Three in my tests.
4. Make a get request to the sidebars endpoint. It requires auth, but the Application Password could be used for basic auth.
5. Confirm that the request loads the same sidebars registered for the Twenty Twenty theme.

Example request made by the editor.

GET /wp-json/wp/v2/sidebars?context=edit&_fields=id,name,description,status,widgets

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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.

[Mamaduka]

@hellofromtonya, I'm unsure what would be the right place to add PHPUnit tests for this change or the actual "unit" we want to test. In the plugin, the feature is covered by e2e tests.

[audrasjb]

@Mamaduka I aded a small comment, but otherwise the logic looks good to me 👍

[Mamaduka]

Thank you, @audrasjb!

Do you have any suggestions about the PHPUnit tests - #3893 (comment)?

[audrasjb]

Concerning unit tests, maybe try to verify that classic sidebars are stored when switching to a block theme?

[Mamaduka]

That makes sense, thank you.

I like the "classic sidebars" term and reference our other discussions from #3902 (comment). Do you think I should change legacy_sidebars to classic_sidebars?

[audrasjb]

Yeah definitely! I think it would be easier to identify what is it: the system used before blocks started to handle WordPress content dataset :)

Yes it is legacy, but "legacy" can also be used for "legacy Classic" functions/classes/methods/concepts (= legacy-legacy stuff 🤪 ). Let's disambiguate when possible! :D

[Mamaduka]

• Fixed docblock.
• Updated the theme mod name.
• Added simple unit tests.

[hellofromtonya]

I like the "classic sidebars" term and reference our other discussions from #3902 (comment). Do you think I should change legacy_sidebars to classic_sidebars?

I agree with @audrasjb that "classic" is preferred over "legacy" 👍

@Mamaduka I'll look at the PHPUnit. Thank you for adding it!

Test Reports: I asked @ironprogrammer to do his magic in testing this PR. If anyone has time, please test and add a test report too.

[hellofromtonya]

@ironprogrammer tested the PR and submitted a Test Report https://core.trac.wordpress.org/ticket/57531#comment:13. Works 👍

[hellofromtonya]

Currently reviewing for commit.

[hellofromtonya]

@Mamaduka I switched the way this PR is tested by:

• Adding a new test class for the new function _wp_block_theme_register_classic_sidebars().
• Testing the "inactive" state in the REST API.

By doing so, it's now testing different paths for this specific change including:

• Does the sidebar register on theme switch after invoking _wp_block_theme_register_classic_sidebars()?
• Does the REST Controller set the sidebar to inactive?
• Does the switch_theme() set the theme mod?

[hellofromtonya]

• Confirmed this PR includes the changes from the GB PR ✅
• It has a test report ✅
• It has unit/integration tests ✅

It's ready for commit (assuming the CI jobs pass this time 🤣 ) 👍

[hellofromtonya]

Committed via https://core.trac.wordpress.org/changeset/55200.