Remove unneeded validation callbacks now we have a default

[joehoyle]

This is a cleanup of all the params to remove the use of rest_validate_request_arg where it's not needed, also some places were doing a custom sanitize callback on enums, which is not needed now either.

[joehoyle]

Added a new patch with tests to cover all the failing cases on the validation. Phew!

[joehoyle]

I also discovered a small issue that I found while writing all the tests - we are accidentally added $taxonomy_exclude as a Schema item, rather than a collection param. This is just in the registration stuff, I added more tests for that specifically fixed it up.

[jnylen0]

There was some whitespace weirdness in 38617.2.diff including a bunch of unintended changes in the test suite. Should be cleaned up in 38617.3.diff. Definitely take a look and make sure I didn't miss anything, but I verified as follows:

• Committed 38617.2.diff to a branch in my local git repository
• Reversed that patch via git show | git apply -R and then applied 38617.3.diff
• git diff -w returns nothing

[rachelbaker]

In 39105:

REST API: Clean-up our validation callbacks and add missing array items properties in our endpoint schemas.

Props joehoyle, jnylen0.
Fixes #38617.