[ty] Pass the generic context through the decorator (#22544)
## Summary
fixes: https://github.com/astral-sh/ty/issues/2336
fixes: https://github.com/astral-sh/ty/issues/2382
This PR fixes a bug where the generic context was not being passed
through the decorator via the `ParamSpec` type variable. For example:
```py
from typing import Callable
def decorator[**P, R](f: Callable[P, R]) -> Callable[P, R]: ...
@decorator
def foo[T](x: T): ...
foo(1)
```
This meant that after the decorator has been applied, the `Callable`
corresponding to the `ParamSpec` type variable did not preserve the
generic context from the decorated function, which lead to `foo` loosing
the generic context.
## Test Plan
Add new mdtest cases.
### Typing conformance result
The false positive that's removed is correct.
The true positives that are removed isn't correct but it's unrelated to
this PR (related to https://github.com/astral-sh/ty/issues/623). The
case is:
```py
def accepts_callable(cb: t.Callable[P, R]) -> t.Callable[P, R]:
return cb
class Class9:
def __init__(self, x: list[T], y: list[T]) -> None: ...
r9 = accepts_callable(Class9)
r9([1], [""]) # E
```
Here, the `r9` call has two arguments and the corresponding parameters
have the _same_ type variable. Pyright specializes using the first
argument and errors on the second argument while ty currently union the
constraints i.e., `T: Unknown | int | str` (`Unknown` because `T` is
inside a `list`).
