Update call_fixture() to work with pytest 8.4. (#166)

Internally the unit tests for pytester access pytest fixtures directly.
This has been [deprecated since
4.x](https://docs.pytest.org/en/stable/deprecations.html#calling-fixtures-directly),
but the project currently contains some workarounds (in
`call_fixture()`) to do this anyway.

In pytest 8.4, the internals changed which breaks the way
`call_fixture()` accesses the underlying fixture. This PR:

 - Updates `call_fixture()` minimally so that it also works with 8.4+.
 - Adds a marker that we should fix this properly.
- Notes a promising direction for doing what we want in a supported
manner.

Resolves #165.

Author: asnare

src/databricks/labs/pytester/fixtures/unwrap.py

@@ -5,6 +5,8 @@
 from typing import TypeVar
 from unittest.mock import MagicMock, create_autospec
 
+import pytest
+
 from databricks.labs.lsql.backends import MockBackend
 from databricks.sdk import WorkspaceClient
 from databricks.sdk.service import iam
@@ -14,13 +16,27 @@
 T = TypeVar('T')
 
 
-def call_fixture(fixture_fn: Callable[..., T], *args, **kwargs) -> T:
-    if not hasattr(fixture_fn, '__pytest_wrapped__'):
-        raise ValueError(f'{fixture_fn} is not a pytest fixture')
-    wrapped = getattr(fixture_fn, '__pytest_wrapped__')
-    if not hasattr(wrapped, 'obj'):
-        raise ValueError(f'{fixture_fn} is not a pytest fixture')
-    return wrapped.obj(*args, **kwargs)
+# Pytest fixtures are not supposed to be called directly, but historically we hack through the countermeasures.
+# TODO: Investigate and fix this if possible, to avoid breakage in future pytest versions.
+# Potential solution: use `pytest.FixtureRequest` & `request.getfixturevalue()` to access fixtures.
+if pytest.version_tuple >= (8, 4):
+
+    def call_fixture(fixture_fn: Callable[..., T], *args, **kwargs) -> T:
+        if not hasattr(fixture_fn, "_get_wrapped_function"):
+            raise ValueError(f'{fixture_fn} is not a pytest fixture')
+        accessor = getattr(fixture_fn, "_get_wrapped_function")
+        wrapped = accessor()
+        return wrapped(*args, **kwargs)
+
+else:
+    # Older versions of pytest use a different mechanism to wrap fixtures.
+    def call_fixture(fixture_fn: Callable[..., T], *args, **kwargs) -> T:
+        if not hasattr(fixture_fn, '__pytest_wrapped__'):
+            raise ValueError(f'{fixture_fn} is not a pytest fixture')
+        wrapped = getattr(fixture_fn, '__pytest_wrapped__')
+        if not hasattr(wrapped, 'obj'):
+            raise ValueError(f'{fixture_fn} is not a pytest fixture')
+        return wrapped.obj(*args, **kwargs)
 
 
 class CallContext: