Add sched_switch_to_idle() helper

When all ready queues are empty, the scheduler should switch
to idle mode and wait for incoming interrupts. This commit
introduces a dedicated helper to handle that transition,
centralizing the logic and improving readbility of the
scheduler path to idle.

Author: vicLin8712

kernel/task.c

@@ -477,6 +477,14 @@ static void sched_idle(void)
         mo_task_wfi();
 }
 
+/* Switch to idle task and return idle task id */
+static inline tcb_t *sched_switch_to_idle(void)
+{
+    kcb->task_current = kcb->harts->task_idle;
+    tcb_t *idle = kcb->harts->task_idle->data;
+    idle->state = TASK_RUNNING;
+    return idle;
+}
 
 /* Efficient Round-Robin Task Selection (Cursor-Based, O(1) Complexity)
  *