Summary

Adds validation and sanitization to the args section of register_rest_route, ensuring the WP REST API validates and sanitizes inputs before using the callback.

This ensures correct HTTP status codes are returned e.g. 400 for when an invalid request is made vs. the current 500 server error, which is inaccurate.

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)