Improve truncate efficiency and fix OBOE in truncate_and_remove_front (#22591)

* Skip walking string for truncate when byte len is <= char limit

* Fix `truncate_and_remove_front` returning string that is `max_chars +
1` in length. Now more consistent with `truncate_and_trailoff` behavior.

* Fix `truncate_and_remove_front` adding ellipsis when max_chars == char
length

Release Notes:

- N/A

crates/tasks_ui/src/modal.rs

@@ -791,7 +791,7 @@ mod tests {
         assert_eq!(
             task_names(&tasks_picker, cx),
             vec![
-                "hello from …th.odd_extension:1:1".to_string(),
+                "hello from …h.odd_extension:1:1".to_string(),
                 "opened now: /dir".to_string()
             ],
             "Second opened buffer should fill the context, labels should be trimmed if long enough"
@@ -820,7 +820,7 @@ mod tests {
         assert_eq!(
             task_names(&tasks_picker, cx),
             vec![
-                "hello from …ithout_extension:2:3".to_string(),
+                "hello from …thout_extension:2:3".to_string(),
                 "opened now: /dir".to_string()
             ],
             "Opened buffer should fill the context, labels should be trimmed if long enough"