[red-knot] Minor follow-up on slice expression inference (#13982)

## Summary

Minor follow-up to #13917 — thanks @AlexWaygood for the post-merge
review.

- Add
SliceLiteralType::as_tuple
- Use .expect() instead of SAFETY
comment
- Match on ::try_from
result
- Add TODO comment regarding raising a diagnostic for `"foo"["bar":"baz"]`

crates/red_knot_python_semantic/resources/mdtest/subscript/string.md

@@ -86,3 +86,14 @@ substring2 = str_instance()[0:5]
 # TODO: Support overloads... Should be `str`
 reveal_type(substring2)  # revealed: @Todo
 ```
+
+## Unsupported slice types
+
+```py
+# TODO: It would be great if we raised an error here. This can be done once
+# we have support for overloads and generics, and once typeshed has a more
+# precise annotation for `str.__getitem__`, that makes use of the generic
+# `slice[..]` type. We could then infer `slice[str, str]` here and see that
+# it doesn't match the signature of `str.__getitem__`.
+"foo"["bar":"baz"]
+```