settings: Show notification when user/project settings fail to parse (#18122)

Closes #16876

We only ever showed parsing errors, but not if something failed to
deserialize.

Basically, if you had a stray `,` somewhere, we'd show a notification
for user errors, but only squiggly lines if you had a `[]` instead of a
`{}`.

The squiggly lines would only show up when there were schema errors.

In the case of `formatter` settings, for example, if someone put in a
`{}` instead of `[]`, we'd never show anything.

With this change we always show a notification if parsing user or
project settings fails.

(Right now, the error message might still be bad, but that's a separate
change)


Release Notes:

- Added a notification to warn users if their user settings or
project-local settings failed to deserialize.

Demo:


https://github.com/user-attachments/assets/e5c48165-f2f7-4b5c-9c6d-6ea74f678683

crates/language/src/language_settings.rs

@@ -1152,6 +1152,13 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_formatter_deserialization_invalid() {
+        let raw_auto = "{\"formatter\": {}}";
+        let result: Result<LanguageSettingsContent, _> = serde_json::from_str(raw_auto);
+        assert!(result.is_err());
+    }
+
     #[test]
     pub fn test_resolve_language_servers() {
         fn language_server_names(names: &[&str]) -> Vec<LanguageServerName> {