Add tests for new list_plugin_distributions() API

- Add test coverage for the new list_plugin_distributions() method
- Verify it returns unwrapped importlib.metadata.Distribution objects
- Confirm DistFacade wrapping only in legacy list_plugin_distinfo()
- Extend existing test_load_setuptools_instantiation test

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: RonnyPfannschmidt

testing/test_pluginmanager.py

@@ -618,6 +618,19 @@ def my_distributions():
     num = pm.load_setuptools_entrypoints("hello")
     assert num == 0  # no plugin loaded by this call
 
+    # Test the new modern API returns unwrapped Distribution objects
+    ret_modern = pm.list_plugin_distributions()
+    assert len(ret_modern) == 1
+    assert len(ret_modern[0]) == 2
+    assert ret_modern[0][0] == plugin
+    assert ret_modern[0][1] == dist  # type: ignore[comparison-overlap]
+
+    # Verify the old API wraps with DistFacade while new API doesn't
+    from pluggy._compat import DistFacade
+
+    assert isinstance(ret[0][1], DistFacade)
+    assert not isinstance(ret_modern[0][1], DistFacade)
+
 
 def test_add_tracefuncs(he_pm: PluginManager) -> None:
     out: list[Any] = []