[ty] Add tests: `types.UnionType` in `isinstance`/`issubclass` (#21537)

## Summary

Add some tests documenting the fact that we don't support
`types.UnionType` in `isinstance`/`issubclass` at the moment.

crates/ty_python_semantic/resources/mdtest/narrow/isinstance.md

@@ -147,6 +147,33 @@ def _(x: int | str | bytes):
         reveal_type(x)  # revealed: (int & Unknown) | (str & Unknown) | (bytes & Unknown)
 ```
 
+## `classinfo` is a `types.UnionType`
+
+Python 3.10 added the ability to use `Union[int, str]` as the second argument to `isinstance()`:
+
+```py
+from typing import Union
+
+IntOrStr = Union[int, str]
+
+reveal_type(IntOrStr)  # revealed: types.UnionType
+
+def _(x: int | str | bytes | memoryview | range):
+    # TODO: no error
+    # error: [invalid-argument-type]
+    if isinstance(x, IntOrStr):
+        # TODO: Should be `int | str`
+        reveal_type(x)  # revealed: int | str | bytes | memoryview[int] | range
+    # TODO: no error
+    # error: [invalid-argument-type]
+    elif isinstance(x, Union[bytes, memoryview]):
+        # TODO: Should be `bytes | memoryview[int]`
+        reveal_type(x)  # revealed: int | str | bytes | memoryview[int] | range
+    else:
+        # TODO: Should be `range`
+        reveal_type(x)  # revealed: int | str | bytes | memoryview[int] | range
+```
+
 ## `classinfo` is a `typing.py` special form
 
 Certain special forms in `typing.py` are aliases to classes elsewhere in the standard library; these

crates/ty_python_semantic/resources/mdtest/narrow/issubclass.md

@@ -200,6 +200,33 @@ def _(x: type[int | str | bytes]):
         reveal_type(x)  # revealed: (type[int] & Unknown) | (type[str] & Unknown) | (type[bytes] & Unknown)
 ```
 
+## `classinfo` is a `types.UnionType`
+
+Python 3.10 added the ability to use `Union[int, str]` as the second argument to `issubclass()`:
+
+```py
+from typing import Union
+
+IntOrStr = Union[int, str]
+
+reveal_type(IntOrStr)  # revealed: types.UnionType
+
+def f(x: type[int | str | bytes | range]):
+    # TODO: No error
+    # error: [invalid-argument-type]
+    if issubclass(x, IntOrStr):
+        # TODO: Should be `type[int] | type[str]`
+        reveal_type(x)  # revealed: type[int] | type[str] | type[bytes] | <class 'range'>
+    # TODO: No error
+    # error: [invalid-argument-type]
+    elif issubclass(x, Union[bytes, memoryview]):
+        # TODO: Should be `type[bytes]`
+        reveal_type(x)  # revealed: type[int] | type[str] | type[bytes] | <class 'range'>
+    else:
+        # TODO: Should be `<class 'range'>`
+        reveal_type(x)  # revealed: type[int] | type[str] | type[bytes] | <class 'range'>
+```
+
 ## Special cases
 
 ### Emit a diagnostic if the first argument is of wrong type