Fix more failure cases of assistant edits (#19653)

* Make `description` optional (since we describe it as optional in the
prompt, and we're currently not showing it)
* Fix fuzzy location bug that neglected the cost of deleting prefixes of
the query.
* Make auto-indent work for single-line edits. Previously, auto-indent
would not occur when overwriting a single line (without inserting or
deleting a newline)

Release Notes:

- N/A