Per-method GPU smoke test: WanCausalModel + DiffusionForcingSFTMethod.
Mirrors test_wan_finetune.py for the diffusion-forcing SFT
(DFSFT) algorithm on the causal Wan transformer. The harness is
intentionally identical so the two tests are easy to compare and so
future per-method tests can copy this template verbatim.
DFSFT samples inhomogeneous timesteps per chunk (chunk_size=3
in the fixture) and is the natural training counterpart of the
WanCausalModel plugin.