Users: Preserve early-authenticated user ID in determine_current_user filter chain

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/28212

When a callback registered on the determine_current_user filter at a priority lower than 10 authenticates a user and returns a user ID, wp_validate_auth_cookie (hooked directly at priority 10) receives that user ID as its $cookie argument.

Because wp_validate_auth_cookie expected a cookie string, it failed parsing and returned false, overriding the user ID determined by the earlier filter callback.

Since we cannot wrap wp_validate_auth_cookie in a new filter callback function (like wp_validate_logged_in_cookie) because changing the hook name in default-filters.php would break backward compatibility for countless stateless API plugins that rely on remove_filter( 'determine_current_user', 'wp_validate_auth_cookie' ).

Instead, this PR adds a safeguard inside wp_validate_auth_cookie() in pluggable.php. If a user ID (non-string or numeric value) is passed, it returns it immediately. If no user ID is passed, it falls through to wp_parse_auth_cookie(), preserving the existing is_ssl() auto-detection logic perfectly.

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