Bring official Volcengine channels in line with the China Mobile asset
isolation model: the platform owns the upstream asset group lifecycle
per (user, channel). asset_group.* APIs are forbidden for clients;
CreateAsset is scoped to the user's managed group (client GroupId is
overwritten), ListAssets is scoped with Filter.GroupIds to the managed
group, and Get/Update/Delete verify ownership via GetAsset before
forwarding, returning not-found for foreign assets.

Generalize the shared managed-group plumbing (GetOrCreateUserAssetGroup,
ScopeManagedAssetRequest, RequireManagedAssetOwnership) and add the
DoubaoVideo-specific group creator (Ark CreateAssetGroup returns the id
in Result.Id; GroupType must be omitted). The DoubaoVideo asset adapter
now serializes the request body from req.Body so platform rewrites take
effect (RawBody was bypassing the scoping) and the V4 signature always
covers the actual payload.

Verified end-to-end: group ops rejected, forged GroupId overwritten by
the auto-created managed group, ListAssets returns only the managed
group, foreign assets hidden from get/delete.

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