Format streamed edits on save (#31623)

Re-enables format on save for agent changes (when the user has that
enabled in settings), except differently from before:
- Now we do the format-on-save in the separate buffer the edit tool
uses, *before* the diff
- This means it never triggers separate staleness
- It has the downside that edits are now blocked on the formatter
completing, but that's true of saving in general.

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <hi@aguz.me>

Cargo.lock

@@ -683,6 +683,7 @@ dependencies = [
  "language_model",
  "language_models",
  "log",
+ "lsp",
  "markdown",
  "open",
  "paths",