The native /api/v3/contents/generations/tasks route was restricted to
the seedance asset channel family (58/60/61); official Volcengine Ark
channels (DoubaoVideo, type 54) could only be reached via the
standardized /v1/video/generations path. Allow type 54 on the native
route in both the distributor and the task retry channel selection,
while keeping it out of the seedance asset binding system.

The taskdoubao adaptor now reuses a pre-parsed task request from the
context (native path) instead of re-parsing the body as TaskSubmitReq,
whose prompt validation would reject the Volcengine-native format.
When req.Prompt is empty (native path), content text items from
metadata are preserved instead of being replaced with an empty prompt;
the standardized path behavior (prompt replaces metadata text) is
unchanged and covered by existing tests.

Co-Authored-By: ZCode <noreply@anthropic.com>