Submit a withdrawal request for an asset.

The request is queued for processing and the amount will be debited from
the user's unlocked balance.

asset_id is always required. Supply either a positive decimal amount
(up to 10 decimal places), or withdraw_all: true without an amount.
Withdraw-all queues exactly one request for the selected asset's full current
unlocked balance, returning one request_id plus asset_id and the exact amount.
It does not include other assets or funds credited after submission.

Both modes reject an existing pending, queued, or requested withdrawal for the
same user and asset with 409 withdrawal_in_progress. Explicit-amount requests
retain replacement of ready requests; withdraw-all rejects ready requests too.
These checks use non-locking reads: simultaneous submissions can still race.