[ty] Support __setitem__ and improve __getitem__ related diagnostics (#19578)

## Summary

Adds validation to subscript assignment expressions.

```py
class Foo: ...

class Bar:
    __setattr__ = None

class Baz:
    def __setitem__(self, index: str, value: int) -> None:
        pass

# We now emit a diagnostic on these statements
Foo()[1] = 2
Bar()[1] = 2
Baz()[1] = 2

```

Also improves error messages on invalid `__getitem__` expressions

## Test Plan

Update mdtests and add more to `subscript/instance.md`

---------

Co-authored-by: David Peter <sharkdp@users.noreply.github.com>
Co-authored-by: David Peter <mail@david-peter.de>

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

@@ -14,7 +14,7 @@ a = NotSubscriptable()[0]  # error: "Cannot subscript object of type `NotSubscri
 class NotSubscriptable:
     __getitem__ = None
 
-# error: "Method `__getitem__` of type `Unknown | None` is not callable on object of type `NotSubscriptable`"
+# error: "Method `__getitem__` of type `Unknown | None` is possibly not callable on object of type `NotSubscriptable`"
 a = NotSubscriptable()[0]
 ```
 
@@ -43,6 +43,18 @@ def _(flag: bool):
     reveal_type(Identity()[0])  # revealed: int | str
 ```
 
+## `__getitem__` with invalid index argument
+
+```py
+class Identity:
+    def __getitem__(self, index: int) -> int:
+        return index
+
+a = Identity()
+# error: [invalid-argument-type] "Method `__getitem__` of type `bound method Identity.__getitem__(index: int) -> int` cannot be called with key of type `Literal["a"]` on object of type `Identity`"
+a["a"]
+```
+
 ## `__setitem__` with no `__getitem__`
 
 ```py
@@ -53,3 +65,45 @@ class NoGetitem:
 a = NoGetitem()
 a[0] = 0
 ```
+
+## Subscript store with no `__setitem__`
+
+```py
+class NoSetitem: ...
+
+a = NoSetitem()
+a[0] = 0  # error: "Cannot assign to object of type `NoSetitem` with no `__setitem__` method"
+```
+
+## `__setitem__` not callable
+
+```py
+class NoSetitem:
+    __setitem__ = None
+
+a = NoSetitem()
+a[0] = 0  # error: "Method `__setitem__` of type `Unknown | None` is possibly not callable on object of type `NoSetitem`"
+```
+
+## Valid `__setitem__` method
+
+```py
+class Identity:
+    def __setitem__(self, index: int, value: int) -> None:
+        pass
+
+a = Identity()
+a[0] = 0
+```
+
+## `__setitem__` with invalid index argument
+
+```py
+class Identity:
+    def __setitem__(self, index: int, value: int) -> None:
+        pass
+
+a = Identity()
+# error: [invalid-assignment] "Method `__setitem__` of type `bound method Identity.__setitem__(index: int, value: int) -> None` cannot be called with a key of type `Literal["a"]` and a value of type `Literal[0]` on object of type `Identity`"
+a["a"] = 0
+```