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.

Only one in-flight withdrawal per asset is allowed. If the user already has a withdrawal in the ready state for the same asset (allocated but not yet executed), it is automatically canceled and replaced by this new request. The previous request's metadata is preserved in the audit log (withdrawal.replaced context) but is not returned in the response.