Improve Media REST API endpoint

[furi3r]

Duplicated queries when querying media_types without files.
await wp.apiFetch({ path: 'wp/v2/media?_envelope&media_type=text'});

Duplicated queries when querying non-existing mime_types.
await wp.apiFetch({ path: 'wp/v2/media?_envelope&mime_type=image/webp'});

Probably related.

[furi3r]

Duplicated queries when querying media_types witouth files.

[gdetassigny]

Thanks for the details @furi3r . I've figured out the issue for this and should have a patch available soon!

## The issue

WP REST API has this logic to re-run the query without the page parameter if we didn't get any posts back from the main one. This is to handle the case where we requested an extra page of a collection that is empty. In that case, we want the API to return an empty collection, but still return the total number of items in that collection. Therefore that extra query gives that total back.

However, it currently doesn't check if we were on the first page already.
In that specific case, the second query would be the exact same as the first one and therefore wouldn't provide any additional value (the total of items would still be zero). This is why we sometimes ends up with a duplicated query.
By checking if we're already on the first page, we can avoid a duplicated query for empty collections.

Note that the trac ticket refers to the error happening on attachments endpoint, however any empty collection of posts will be affected (This is why some tests that were previously checking for two queries and had a "FIXME" comment, have been updated to one).

## How to test

• Query Monitor plugin should be enabled to detect any duplicate queries.
• Open the gutenberg editor
• Open the JS console
• Type await wp.apiFetch({ path: 'wp/v2/media?_envelope&media_type=text'}
• The object returned should contain the number of duplicated queries. On trunc, there will be a duplicate which this patch will fix.

Trac ticket: https://core.trac.wordpress.org/ticket/55677

[gdetassigny]

I've added a patch to fix this.
Something worth noting : while this issue was reported about the attachments rest endpoint, this actually is a potential issue for any post type. It's probably more apparent on attachments because they're more likely to have a completely empty collection.

Can we also interrogate the X-WP-TotalPages and X-WP-Total headers in our tests here to have assertions that they still match our expected results.

Thanks @TimothyBJacobs . I've made some changes now:

• first test for an empty collection now also asserts these headers
• second test checks the appropriate _invalid page number_ error is returned (headers cannot be verify because they don't get set in this specific case, ​the error is returned right before the headers would be set)

Thanks @gabriel-detassigny, this looks good to me!

[spacedmonkey]

In 53498:

REST API: Avoid duplicated query in post collections.

Avoid duplicated query when retrieving empty posts collections by adding a check if the page is more than 1.

Props furi3r, gdetassigny, TimothyBlynJacobs, spacedmonkey.
Fixes #55677.

Committed ​https://github.com/WordPress/wordpress-develop/commit/219927c11b001fad95e1f2ee6b17d132b8d00af2.

[spacedmonkey]

Early draft of a dev note can be found ​here.