Feat+Refactor: Introduce a wrapper for scalar values and refactor with it

[mpyw]

Minor Breaking Changes

• Assigning NULL throws InvalidArgumentException instead of PDOException without running queries.

     public function testAssigningNullThrowsExceptionOnNative(): void
     {
-        $this->expectException(PDOException::class);
-        $this->expectExceptionMessage("SQLSTATE[42000]: Syntax error or access violation: 1231 Variable 'foreign_key_checks' can't be set to the value of 'NULL'");
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('The value must be a scalar or Mpyw\LaravelMySqlSystemVariableManager\ValueInterface instance.');
 
         $this->onNativeConnection(function (MySqlConnection $db) {
             $db->setSystemVariable('foreign_key_checks', null);
             $db->getPdo();
         });
     }
 
     public function testAssigningNullThrowsExceptionOnEmulation(): void
     {
-        $this->expectException(PDOException::class);
-        $this->expectExceptionMessage("SQLSTATE[42000]: Syntax error or access violation: 1231 Variable 'foreign_key_checks' can't be set to the value of 'NULL'");
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('The value must be a scalar or Mpyw\LaravelMySqlSystemVariableManager\ValueInterface instance.');
 
         $this->onEmulatedConnection(function (MySqlConnection $db) {
             $db->setSystemVariable('foreign_key_checks', null);
             $db->getPdo();
         });
     }

• Assigning NULL throws InvalidArgumentException even if the PDO connection is not established.

-    public function testAssigningNullDoesNotThrowOnUnresolvedConnection(): void
+    public function testAssigningNullThrowsOnUnresolvedNativeConnection(): void
     {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('The value must be a scalar or Mpyw\LaravelMySqlSystemVariableManager\ValueInterface instance.');
+
         $this->onNativeConnection(function (MySqlConnection $db) {
             $db->setSystemVariable('foreign_key_checks', null);
         });
+    }
+
+    public function testAssigningNullThrowsOnUnresolvedEmulatedConnection(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('The value must be a scalar or Mpyw\LaravelMySqlSystemVariableManager\ValueInterface instance.');
+
         $this->onEmulatedConnection(function (MySqlConnection $db) {
             $db->setSystemVariable('foreign_key_checks', null);
         });
-        $this->assertTrue(true);
     }