Add idle task per hart at boot to prevent panic on no runnable tasks

To prevent kernel panic during startup when some harts may not have any
runnable tasks assigned, add an idle task for each hart. The idle task
runs an infinite loop calling mo_task_wfi(), ensuring the hart remains
in a low-power wait state instead of causing a panic due to lack of
tasks.

This guarantees that every hart has at least one task to execute
immediately after boot, improving system robustness and stability on
SMP setups.

Author: visitorckw

kernel/main.c

@@ -5,6 +5,12 @@
 
 #include "private/error.h"
 
+static void idle_task(void)
+{
+    while (1)
+        mo_task_wfi();
+}
+
 static volatile bool finish = false;
 static spinlock_t finish_lock = SPINLOCK_INITIALIZER;
 
@@ -47,6 +53,8 @@ int32_t main(int32_t hartid)
     }
     spin_unlock(&finish_lock);
 
+    mo_task_spawn(idle_task, DEFAULT_STACK_SIZE);
+
     /* Verify that the application created at least one task.
      * If 'kcb->task_current' is still NULL, it means mo_task_spawn was never
      * successfully called.