Only check a filter callback's own returns in AlwaysReturnInFilter

[GaryJones]

AlwaysReturnInFilter looked at every return in a filter callback's body, including returns that belong to a nested closure or an anonymous class method. It also treated a return as conditional whenever there was an if anywhere above it, even an if outside the callback. That gave two false positive errors and one false negative.

An intentional return; in a nested closure was reported as the filter returning void:

add_filter( 'the_content', function( $content ) {
	add_action( 'wp_footer', function() {
		return;
	} );
	return $content; // VoidReturn error on the filter.
} );

A callback wrapped in a function_exists() check that returns only from try and catch was reported as missing a return. The same function without the wrapper passed:

if ( ! function_exists( 'my_filter' ) ) {
	function my_filter( $value ) { // MissingReturnStatement error.
		try {
			return compute( $value );
		} catch ( Exception $e ) {
			return $value;
		}
	}
}
add_filter( 'my_filter', 'my_filter' );

A callback that never returns passed if a nested closure inside it did:

add_filter( 'my_filter', function( $items ) { // Not flagged.
	$items = array_map( function( $item ) {
		return $item;
	}, $items );
} );

The sniff now skips any return whose nearest enclosing function or closure is not the callback, using Conditions::getLastCondition() from PHPCSUtils. When it decides whether a return is conditional, it only counts if statements inside the callback.

The last case means code that passed before can now get a MissingReturnStatement error. That is the correct result, but it may be worth a line in the changelog.

Two related gaps are left for another time. A return that appears only in an else still counts as unconditional, because the sniff only treats if as a control structure (see #520). hasTerminatingStatement() also still sees an exit or throw inside a nested closure. That only changes whether a callback with no direct return gets the warning or the error.

This came up while reviewing #904 and #905.