Fix: Improve table existence check and test coverage

Author: prab18hat

features/core.feature

@@ -27,6 +27,7 @@ Feature: Manage WordPress installation
       """
       WordPress is not installed. Missing tables:
       """
+    And STDERR should contain: wp_
 
     When I try `wp core is-installed --network`
     Then the return code should be 1

src/Core_Command.php

@@ -403,17 +403,26 @@ public function is_installed( $args, $assoc_args ) {
 			// Check if all core tables exist
 			foreach ( $tables as $table ) {
 				$table_name = $wpdb->prefix . $table;
-				$result = $wpdb->get_var( $wpdb->prepare(
-					"SHOW TABLES LIKE %s", $table_name
-				) );
+				$result = $wpdb->get_var(
+					$wpdb->prepare(
+						'SHOW TABLES LIKE %s',
+						$table_name
+					)
+				);
 
 				if ( $result !== $table_name ) {
 					$missing_tables[] = $table_name;
 				}
 			}
 
 			if ( ! empty( $missing_tables ) ) {
-				WP_CLI::error( "WordPress is not installed. Missing tables: " . implode( ', ', $missing_tables ), 1 );
+				WP_CLI::error(
+					sprintf(
+						'WordPress is not installed. Missing tables: %s',
+						implode( ', ', $missing_tables )
+					),
+					1
+				);
 			}
 
 			WP_CLI::halt( 0 );