The pipeline class is selected from the request mode, which can disagree with
the model's real capability (transformer input channels), causing a hard
channel-mismatch crash. Detect and degrade gracefully for Wan:

- ti2v/i2v request on a t2v model (transformer in_channels=16): rebuild as a
  plain WanPipeline and run t2v with the keyframe dropped.
- t2v request on an i2v model (in_channels=36): rebuild as
  WanImageToVideoPipeline (image_encoder/processor are optional) and seed a
  neutral gray frame so the prompt still drives the clip.

Both rebuild a sibling pipeline reusing the SAME components, so fused
acceleration and per-request LoRAs on the shared transformer carry over with no
reload; the view is cached on the pipe so repeated clips reuse it and
_sync_video_loras' adapter dedup stays intact. Helpers: _wan_in_channels(),
_maybe_t2v_fallback(), _maybe_i2v_fallback().
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>