REST API: Terms controller doesn't respect taxonomy's default query args

[Bernhard Reiter]

It is possible to supply a default set of default query args to ​`register_taxonomy()` which will be used when querying a list of terms -- for example, orderby in order to specify how the resulting list of terms should be sorted.

However, those default query args are not respected by the Terms REST API controller, if the request doesn't contain a post ID.

[Bernhard Reiter]

The controller's get_items() method has ​two code branches:

1. If a post ID is given in the network request, get_items() uses ​`wp_get_object_terms()` to get the terms associated with the given post. That function includes ​code to respect the default query args specified when the taxonomy was registered.
2. If no post ID is given, it uses ​`get_terms()`, which does not respect the default query args.

For the practical impact, more background, and a fix in the GB repo, see ​https://github.com/WordPress/gutenberg/issues/65052 and ​https://github.com/WordPress/gutenberg/pull/67154.

See ​https://github.com/WordPress/gutenberg/pull/67154. More details to follow.

TODO:

• [ ] Test coverage

Trac ticket: https://core.trac.wordpress.org/ticket/62500

I'll go ahead and merge this per ​https://github.com/WordPress/gutenberg/pull/67154#pullrequestreview-2454037832:

[T]he Core backport in ​https://github.com/WordPress/wordpress-develop/pull/7848 also makes perfect sense. The REST controller should apply the taxonomy's default args.

[Bernhard Reiter]

In 59458:

REST API: Terms: Respect taxonomy's default query args.

It is possible to supply a set of default query args to register_taxonomy() which will be used when querying a list of terms -- for example, orderby in order to specify how the resulting list of terms should be sorted.

The Terms REST API controller previously respected these default query args only if the request included a post ID. This changeset makes it so that the default args will also be respected if no post ID is provided.

Props bernhard-reiter, jsnajdr.
Fixes #62500.

Committed to Core in https://core.trac.wordpress.org/changeset/59458.