code completion for pytest fixtures

### Describe the Bug

```py
import pytest
from car import Car

@pytest.fixture
def my_car():
return Car(50)

def test_car_accelerate(my_car):
my_car.accelerate()
assert my_car.speed == 55
```

when input my_car, provide code completion.

ref https://www.jetbrains.com/help/pycharm/2026.2/pytest.html#pytest-fixtures

### Sandbox Link

_No response_

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

_No response_

Start with the pytest fixture example in the issue and the linked PyCharm fixture reference, then trace Pyrefly's completion handling for test-function parameters. Done means typing my_car in the example provides completion for the Car instance, including accelerate and speed.

Written by the indexing model from the issue text.