settings: Make "auto" and "language_server" valid format steps (#40113)

Follow up for: #39983 and
https://github.com/zed-industries/zed/pull/40040#issuecomment-3393902691

Previously it was possible to have formatting done using prettier or
language server using `"formatter": "auto"` and specify code actions to
apply on format using the `"code_actions_on_format"` setting. However,
post #39983 this is no longer possible due to the removal of the
`"code_actions_on_format"` setting. To rectify this regression, this PR
makes it so that the `"auto"` and `"language_server"` strings that were
previously only allowed as top level values on the `"formatter"` key,
are now allowed as format steps like so:
```json
{
      "formatter": ["auto", "language_server"]
}
```

Therefore to replicate the previous behavior using `"auto"` and
`"code_actions_on_format"` you can use the following configuration:

```json
{
      "formatter": [{"code_action": ...}, "auto"]
}
```

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/agent2/src/tools/edit_file_tool.rs

@@ -790,7 +790,7 @@ mod tests {
                 store.update_user_settings(cx, |settings| {
                     settings.project.all_languages.defaults.format_on_save = Some(FormatOnSave::On);
                     settings.project.all_languages.defaults.formatter =
-                        Some(language::language_settings::SelectedFormatter::Auto);
+                        Some(language::language_settings::FormatterList::default());
                 });
             });
         });