transformer: automatically add `assert_never` to `match` if there is no `_` case

```py
def f(data: int):
match data:
case int(): ...
```
becomes
```py

def f(data: int):
match data:
case int(): ...
case _:
assert_never(data)
```

Start from the Python transformation example in the issue and locate the transformer entry point and its existing tests. Confirm that a match without a wildcard case receives the shown assert_never branch, while the issue's expected output serves as the completion check.

Written by the indexing model from the issue text.