[CI] Reorganize the L1 L2 use cases and add markers (#2449)

Signed-off-by: zhumingjue <zhumingjue@huawei.com>

tests/engine/test_output_modality.py

@@ -12,6 +12,7 @@ import pytest
 import torch
 
 # ── Load modules without triggering vllm_omni.__init__ ─────────────
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
 
 _ENGINE_DIR = Path(__file__).resolve().parents[2] / "vllm_omni" / "engine"