Fix test_protected_namespace_defaults with -Wdefault

This test failed with "pytest -Wdefault", which distros usually enable
for downstream testing to avoid surprise failures.

Use proper `pytest.warns()` method instead of incorrect
`pytest.raises()`.

See: https://mgorny.pl/articles/downstream-testing-python-packages.html#assuming-werror-catching-warnings-as-exceptions

tests/test_settings.py

@@ -2175,7 +2175,7 @@ class Model(BaseSettings):
             model_prefixed_field: str
 
     # pydantic-settings default
-    with pytest.raises(
+    with pytest.warns(
         UserWarning, match='Field "settings_prefixed_field" in Model1 has conflict with protected namespace "settings_"'
     ):