Fix: Validate rest_route Query Var to Ensure String Type

[geekofshire]

This PR introduces an early return validation in rest_api_loaded( to ensure that the rest_route query variable is always a string. If the rest_route is not a string, the function now returns a WP_Error, allowing for better error handling and maintaining proper REST API response behaviour.

Changes:

• Added an early return if rest_route is not a string.
• Replaced wp_die() with a WP_Error response to improve flexibility.

Trac ticket: https://core.trac.wordpress.org/ticket/62932

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