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[ty] Support isinstance() and issubclass() narrowing when the second argument is a typing.py stdlib alias (#21391)

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## Summary

A followup to https://github.com/astral-sh/ruff/pull/21386

## Test Plan

New mdtests added
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Alex Waygood 2025-11-11 21:09:24 +00:00 • committed by GitHub
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crates/ty_python_semantic/resources/mdtest/narrow/isinstance.md
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@ -147,6 +147,25 @@ def _(x: int | str | bytes):
reveal_type(x) # revealed: (int & Unknown) | (str & Unknown) | (bytes & Unknown)
```
## `classinfo` is a `typing.py` special form
Certain special forms in `typing.py` are aliases to classes elsewhere in the standard library; these
can be used in `isinstance()` and `issubclass()` checks. We support narrowing using them:
```py
import typing as t
def f(x: dict[str, int] | list[str], y: object):
if isinstance(x, t.Dict):
reveal_type(x) # revealed: dict[str, int]
else:
reveal_type(x) # revealed: list[str]
if isinstance(y, t.Callable):
# TODO: a better top-materialization for `Callable`s (https://github.com/astral-sh/ty/issues/1426)
reveal_type(y) # revealed: () -> object
```
## Class types
```py