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/task/src/task_template.rs

@@ -570,7 +570,7 @@ mod tests {
                 spawn_in_terminal.label,
                 format!(
                     "test label for 1234 and …{}",
-                    &long_value[..=MAX_DISPLAY_VARIABLE_LENGTH]
+                    &long_value[long_value.len() - MAX_DISPLAY_VARIABLE_LENGTH..]
                 ),
                 "Human-readable label should have long substitutions trimmed"
             );