This changeset fixes a warning where `strip_tags()` expected its first parameter to be a string rather than an array. It contains the following changes:
- Removal of sanitize_callback to allow the REST API to handle the validation natively, this also causes the proper error to be output for `?slug[0][1]=2` that it's an invalid value.
- Ensure that `wp_parse_list()` only returns a single-dimensioned array, even if passed a multi-dimension array, which fits the functions expected use case and resolves warnings in code that expects the function to return a single-dimensioned array.
Props dd32, TimothyBlynJacobs.
Fixes#55838.
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