Refactor mo_task_spawn() for the new scheduler

This commit refactors mo_task_spawn() to align with the new O(1) scheduler
design. The task control block (tcb_t) embeds its list node during task
creation.

The enqueue operation is moved inside a critical section to guarantee
consistent enqueuing process during task creation.

The “first task assignment” logic is removed because first task has been
assigned to system idle task as previous commit mentioned.

Author: vicLin8712

kernel/task.c

@@ -747,8 +747,11 @@ int32_t mo_task_spawn(void *task_entry, uint16_t stack_size_req)
     tcb->id = kcb->next_tid++;
     kcb->task_count++; /* Cached count of active tasks for quick access */
 
-    if (!kcb->task_current)
-        kcb->task_current = node;
+    /* Binding ready queue node */
+    tcb->rq_node.data = tcb;
+
+    /* Push node to ready queue */
+    sched_enqueue_task(tcb);
 
     CRITICAL_LEAVE();
 
@@ -762,7 +765,6 @@ int32_t mo_task_spawn(void *task_entry, uint16_t stack_size_req)
 
     /* Add to cache and mark ready */
     cache_task(tcb->id, tcb);
-    sched_enqueue_task(tcb);
 
     return tcb->id;
 }