Pytest 8.3.3 breaks fixture registration for class based plugins when fixtures are dynamically added to the instance only

[lovetheguitar]

In c6a5290 the detection of fixtures was changed in a way that broke our dynamic fixture registration. Before fixtures were looked up on the instance, but now they will be looked up on the class of a plugin instance.
Relevant line.

Thus I'm wondering what the officially supported plugins one may feed into

pytest/src/_pytest/config/__init__.py

Line 489 in 7add2af

def register(self, plugin: _PluggyPlugin, name: str | None = None) -> str | None:

are?

Reproduction:
test_demo.py

"""Succeeds for <8.3.3, but fails since."""
import pytest


pytest_plugins = ["pytester"]


def test_demo_fixture_present_on_plugin_instance_only_are_not_registered_anymore(pytester: pytest.Pytester) -> None:
    pytester.makeconftest(
        """
        from typing import Callable
        import types
        import pytest
        
        class PluginContainer:
            def add_fixture(
                self, fixture_function: Callable[..., object], fixture_name: str
            ) -> None:
                '''Add pytest fixture function with specified name to Plugin Container.
        
                .. note:: For pytest to find the fixture its apparently important that methods are
                    bound to the instance!
                :param fixture_function: a pytest fixture object
                :param fixture_name: the desired name of the fixture
                '''
                setattr(self, fixture_name, types.MethodType(fixture_function, self))
                
        def pytest_configure(config: pytest.Config) -> None:
            plugin_container = PluginContainer()
            
            @pytest.fixture
            def demo_fixture(self, request):
                ...
                
            plugin_container.add_fixture(demo_fixture, "fixture_name")
    
            config.pluginmanager.register(plugin_container, __name__)
    """)


    result = pytester.runpytest("--fixtures")
    assert result.ret == pytest.ExitCode.OK
    result.stdout.fnmatch_lines(["*fixture_name*"])

[RonnyPfannschmidt]

looks like a "bug" as we never explicitly supported dropping dynamicfixtures into a object instances
we should look into __dict__ aswell when applicable

[RonnyPfannschmidt]

i think in this particular case we may use inspect.getattr_static