[ty] Project exact-length narrowing through TypeVars (#26852)
## Summary
Support exact length narrowing also for typevars. Code like this was
previously rejected.
```python
T = TypeVar("T", tuple[int, int], tuple[int, int, str])
def f(x: T) -> None:
if len(x) == 2: ...
elif len(x) == 3: ...
else: assert_never(x) # `Never` and `T@f_typevar` are not equivalent types
```
## Test Plan
New mdtests
