Change PDO to singleton instance

- Add SQLite throwaway DB implementation
- Add integration tests for Custom class

src/Custom.php

@@ -4,6 +4,7 @@ namespace BitBadger\PDODocument;
 
 use BitBadger\PDODocument\Mapper\Mapper;
 use PDO;
+use PDOException;
 use PDOStatement;
 
 /**
@@ -22,7 +23,13 @@ class Custom
     public static function &runQuery(string $query, array $parameters): PDOStatement
     {
         $debug = defined('PDO_DOC_DEBUG_SQL');
-        $stmt  = Configuration::dbConn()->prepare($query);
+        try {
+            $stmt = Configuration::dbConn()->prepare($query);
+        } catch (PDOException $ex) {
+            $keyword = explode(' ', $query, 2)[0];
+            throw new DocumentException("Error executing $keyword statement: " . Configuration::dbConn()->errorCode(),
+                previous: $ex);
+        }
         foreach ($parameters as $key => $value) {
             if ($debug) echo "<pre>Binding $value to $key\n</pre>";
             $dataType = match (true) {
@@ -115,7 +122,7 @@ class Custom
      * @return mixed|false|T The scalar value if found, false if not
      * @throws DocumentException If any is encountered
      */
-    public static function scalar(string $query, array $parameters, Mapper $mapper, ?PDO $pdo = null): mixed
+    public static function scalar(string $query, array $parameters, Mapper $mapper): mixed
     {
         try {
             $stmt = &self::runQuery($query, $parameters);