feat: add UserId value object and update ClassifiedAd to use it

tests/test_user_id.py

@@ -0,0 +1,25 @@
+from uuid import UUID, uuid4
+
+from domain.marketplace.user_id import UserId
+
+
+def test_user_id_creation():
+    user_id = UserId(value=uuid4())
+    assert isinstance(user_id.value, UUID)
+
+
+def test_user_id_is_immutable():
+    user_id = UserId(value=uuid4())
+
+    try:
+        user_id.value = uuid4()  # type: ignore
+        assert False, "Should have raised FrozenInstanceError"
+    except AttributeError:
+        pass
+
+
+def test_user_id_equality():
+    uid = uuid4()
+    user_id1 = UserId(value=uid)
+    user_id2 = UserId(value=uid)
+    assert user_id1 == user_id2