Reduce fix to minimal changes: only z.date() handling logic and basic test

Co-authored-by: BenLorantfy <4398635+BenLorantfy@users.noreply.github.com>

Author: Copilot

package-lock.json

@@ -1,8 +1,6 @@
-export function assert(
-  condition: unknown,
-  message = 'Assertion failed'
-): asserts condition {
-  if (!condition) {
-    throw new Error(`[nestjs-zod] ${message}`)
-  }
+export function assert(condition: unknown, message: string = 'Assertion failed'): asserts condition {
+    if (!condition) {
+        throw new Error(`[nestjs-zod] ${message}`);
+    }
 }
+