support yield in pytest

### Describe the Bug

```py
from pytest import fixture as reusable

@reusable
def user_id():
yield 42

def test_lookup(user_id):
assert user_id > 0

```

this is not supported yet.

### Sandbox Link

_No response_

### (Only applicable for extension issues) IDE Information

_No response_

Use the reported pytest fixture example as the reproduction: a fixture aliased from pytest.fixture yields 42 and should provide an integer to test_lookup. Locate Pyrefly's handling of pytest fixtures and run the relevant fixture or type-checker tests; done means this example type-checks with user_id inferred as a usable value.

Written by the indexing model from the issue text.