test(msgspec): migrate test_msgspec from contrib to canonical location

Move tests/unit/test_contrib/test_msgspec.py to tests/unit/test_dto/
since the test file already imports from the canonical
``litestar.dto`` namespace. The orphaned ``int_factory`` fixture
moves to tests/unit/test_dto/conftest.py (its only consumer was
test_msgspec.py); the now-empty tests/unit/test_contrib/conftest.py
is removed.

Refs #4723

Author: cofin

tests/unit/test_contrib/conftest.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from collections.abc import Collection, Generator
+from collections.abc import Callable, Collection, Generator
 from typing import Any, TypeVar, get_type_hints
 
 import pytest
@@ -19,6 +19,11 @@
 T = TypeVar("T", bound=Model)
 
 
+@pytest.fixture
+def int_factory() -> Callable[[], int]:
+    return lambda: 2
+
+
 @pytest.fixture
 def ModelDataDTO(use_experimental_dto_backend: bool) -> type[AbstractDTO]:
     class DTOCls(AbstractDTO[Model]):