[ty] Preserve structural type[Protocol] through generic substitution
## Generic identity loses `type[P]`
**Report entries:**
- [psycopg `psycopg/psycopg/_adapters_map.py:143`](https://github.com/psycopg/psycopg/blob/6a52e77dc38827214b294759823a072841b4819f/psycopg/psycopg/_adapters_map.py#L143)
- [psycopg `psycopg/psycopg/_adapters_map.py:182`](https://github.com/psycopg/psycopg/blob/6a52e77dc38827214b294759823a072841b4819f/psycopg/psycopg/_adapters_map.py#L182)
This is an implementation regression rather than a newly enforced protocol rule: passing `type[P]` through an identity-shaped generic function returns a value displayed as the same `type[P]`, but the PR then rejects assigning it back to `type[P]`.
```python
# Merge base: no diagnostic
# PR: error[invalid-assignment] Object of type `type[P]` is not assignable to `type[P]`
from typing import Protocol, TypeVar
class P(Protocol):
value: int
T = TypeVar("T")
def identity(cls: type[T]) -> type[T]:
return cls
def f(cls: type[P]) -> None:
cls = identity(cls)
```
ruff
b59f24f7 - [ty] Preserve structural type[Protocol] through generic substitution
