diffusers' Model.set_attention_backend() doesn't just set per-processor backends
— it ALSO flips a process-wide active backend (attention_dispatch's
_active_backend). The video path sets that to flash-attn for the Wan transformer;
image and video share the nvidia-engine process, so the global stayed flash and
leaked to the next image model. Z-Image's transformer sets no backend of its own
(passes backend=None → uses the global) and its attention is masked, so it
crashed with "`attn_mask` is not supported for flash-attn 2" → image/environment
generation 400. reset_attention_backend() clears per-processor backends but NOT
the global, so it didn't help.

Fix: restore the diffusers global backend to the env default (native/SDPA)
(a) before every image generation — bulletproof against a leaked flash backend —
and (b) in the video pipeline teardown (_free_pipeline_vram), so it can't persist
after a video pipe is freed. Masked image attention (SDPA) now always works; the
video transformer keeps its own per-processor backend.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RdMufYvtTbtGDWsiZVoXce