Avoid marking `InitVar` as a typing-only annotation (#9688)
## Summary
Given:
```python
from dataclasses import InitVar, dataclass
@dataclass
class C:
i: int
j: int = None
database: InitVar[DatabaseType] = None
def __post_init__(self, database):
if self.j is None and database is not None:
self.j = database.lookup('j')
c = C(10, database=my_database)
```
We should avoid marking `InitVar` as typing-only, since it _is_ required
by the dataclass at runtime.
Note that by default, we _already_ don't flag this, since the
`@dataclass` member is needed at runtime too -- so it's only a problem
with `strict` mode.
Closes https://github.com/astral-sh/ruff/issues/9666.
