Add sched_switch_to_idle() helper in the scheduler

Previously, when all ready queues were empty, the scheduler
would trigger a kernel panic. This condition should instead
transition into the idle task rather than panic.

The new sched_switch_to_idle() helper centralizes this logic,
making the path to idle clearer and more readable.

Author: vicLin8712

kernel/task.c

@@ -515,7 +515,7 @@ uint16_t sched_select_next_task(void)
     /* Check out bitmap */
     uint32_t bitmap = kcb->harts->ready_bitmap;
     if (unlikely(!bitmap))
-        panic(ERR_NO_TASKS);
+        return sched_switch_to_idle()->id;
 
     /* Find top priority ready queue */
     int top_prio_level = 0;