Qwen3.5/3.8 text-only arms: register Qwen3_5[Moe]ForCausalLM, resolve the backbone prefix, refuse unsupported MoE layouts (#490)

Qwen3.5/3.8 text-only arms: register Qwen3_5[Moe]ForCausalLM, resolve the backbone prefix, refuse unsupported MoE layouts (#490) #2816