Promote a draft task out of the pre-dispatch draft state

Transitions a draft task (#1240 — created with attachments still uploading) to its normal dispatch-eligible status: offered if it was offered to an agent, pending if it has an owning agent (the common case — UI-composer tasks default to Lead), otherwise unassigned. Called by the UI composer once its attachment upload batch settles, whether every file uploaded, some failed, or all failed — a draft must never be stranded by an upload error. Idempotent: calling it on a task that already left draft returns the current task unchanged rather than erroring, so a retried request is safe.