`Container.__contains__` should be generic

```py
@runtime_checkable
class Container(Protocol[_T_co]):
# This is generic more on vibes than anything else
@abstractmethod
def __contains__(self, x: object, /) -> bool: ...
```
>>>
```py
@runtime_checkable
class Container(Protocol[_T_co]):
@abstractmethod
# must implement safe variance
def __contains__(self, x: _T_co, /) -> bool: ... # type: ignore[unsafe-variance]
```

The issue provides no file or test path; start by locating the Python typing definition of the Container protocol and reviewing its generic behavior. Done means __contains__ uses the type parameter with safe variance, with regression coverage for the updated signature.

Written by the indexing model from the issue text.