[ty] add support for mapped union and intersection subscript loads (#18846)

## Summary

Note this modifies the diagnostics a bit. Previously performing
subscript access on something like `NotSubscriptable1 |
NotSubscriptable2` would report the full type as not being
subscriptable:

```
[non-subscriptable] "Cannot subscript object of type `NotSubscriptable1 | NotSubscriptable2` with no `__getitem__` method"
```

Now each erroneous constituent has a separate error:

```
[non-subscriptable] "Cannot subscript object of type `NotSubscriptable2` with no `__getitem__` method"
[non-subscriptable] "Cannot subscript object of type `NotSubscriptable1` with no `__getitem__` method"
```

Closes https://github.com/astral-sh/ty/issues/625

## Test Plan

 mdtest

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/ty_python_semantic/resources/mdtest/subscript/class.md

@@ -63,12 +63,12 @@ def _(flag: bool):
 
     else:
         class Spam: ...
-    # error: [possibly-unbound-implicit-call] "Method `__class_getitem__` of type `<class 'Spam'> | <class 'Spam'>` is possibly unbound"
-    # revealed: str
+    # error: [non-subscriptable] "Cannot subscript object of type `<class 'Spam'>` with no `__class_getitem__` method"
+    # revealed: str | Unknown
     reveal_type(Spam[42])
 ```
 
-## TODO: Class getitem non-class union
+## Class getitem non-class union
 
 ```py
 def _(flag: bool):
@@ -80,8 +80,7 @@ def _(flag: bool):
     else:
         Eggs = 1
 
-    a = Eggs[42]  # error: "Cannot subscript object of type `<class 'Eggs'> | Literal[1]` with no `__getitem__` method"
+    a = Eggs[42]  # error: "Cannot subscript object of type `Literal[1]` with no `__getitem__` method"
 
-    # TODO: should _probably_ emit `str | Unknown`
-    reveal_type(a)  # revealed: Unknown
+    reveal_type(a)  # revealed: str | Unknown
 ```

crates/ty_python_semantic/resources/mdtest/subscript/tuple.md

@@ -188,3 +188,30 @@ class C(Tuple): ...
 # revealed: tuple[<class 'C'>, <class 'tuple[Unknown, ...]'>, <class 'Sequence[Unknown]'>, <class 'Reversible[Unknown]'>, <class 'Collection[Unknown]'>, <class 'Iterable[Unknown]'>, <class 'Container[Unknown]'>, typing.Protocol, typing.Generic, <class 'object'>]
 reveal_type(C.__mro__)
 ```
+
+### Union subscript access
+
+```py
+def test(val: tuple[str] | tuple[int]):
+    reveal_type(val[0])  # revealed: str | int
+
+def test2(val: tuple[str, None] | list[int | float]):
+    reveal_type(val[0])  # revealed: str | int | float
+```
+
+### Union subscript access with non-indexable type
+
+```py
+def test3(val: tuple[str] | tuple[int] | int):
+    # error: [non-subscriptable] "Cannot subscript object of type `int` with no `__getitem__` method"
+    reveal_type(val[0])  # revealed: str | int | Unknown
+```
+
+### Intersection subscript access
+
+```py
+from ty_extensions import Intersection, Not
+
+def test4(val: Intersection[tuple[str], tuple[int]]):
+    reveal_type(val[0])  # revealed: str & int
+```