Add DateTimeType validation to architecture tests

- Included `DateTimeType` in structure tests to ensure that `PhpTypedValues\DateTime` classes extend `DateTimeType`, are final, and read-only.

Author: georgedeploy

tests/Arch/StructureTest.php

@@ -2,6 +2,7 @@
 
 declare(strict_types=1);
 
+use PhpTypedValues\Code\DateTime\DateTimeType;
 use PhpTypedValues\Code\Float\FloatType;
 use PhpTypedValues\Code\Integer\IntType;
 use PhpTypedValues\Code\String\StrType;
@@ -32,3 +33,10 @@
     ->toBeClasses()
     ->toBeFinal()
     ->toBeReadonly();
+
+arch('Datetime classes are final and read-only')
+    ->expect('PhpTypedValues\DateTime')
+    ->toExtend(DateTimeType::class)
+    ->toBeClasses()
+    ->toBeFinal()
+    ->toBeReadonly();