Fix PHPStan error about non-callable being returned

> phpstan: Method WP_AI_Client_Prompt_Builder::get_builder_callable() should return callable(): mixed but returns array{WordPress\AiClient\Builders\PromptBuilder, string}.

Author: westonruter

src/wp-includes/ai-client/class-wp-ai-client-prompt-builder.php

@@ -392,7 +392,8 @@ private static function is_generating_method( string $name ): bool {
 	protected function get_builder_callable( string $name ): callable {
 		$camel_case_name = $this->snake_to_camel_case( $name );
 
-		if ( ! is_callable( array( $this->builder, $camel_case_name ) ) ) {
+		$method = array( $this->builder, $camel_case_name );
+		if ( ! is_callable( $method ) ) {
 			throw new BadMethodCallException(
 				sprintf(
 					/* translators: 1: Method name. 2: Class name. */
@@ -403,7 +404,7 @@ protected function get_builder_callable( string $name ): callable {
 			);
 		}
 
-		return array( $this->builder, $camel_case_name );
+		return $method;
 	}
 
 	/**