Eligibility Check

Mandatory gate between the transaction selection screen and the dispute reasons screen. Validates two things: each selected transaction is individually eligible for self-service dispute submission, and all selected transactions share the same payment rail — ACH and Debit cannot be mixed in a single dispute. On failure, returns one structured error per failed rule, each with a machine-readable code and a human-readable message, that Lumin surfaces inline on the transaction selection screen; the member stays in the flow and can adjust their selection. Transaction-scoped codes (RAIL_MISMATCH, TRANSACTION_NOT_FOUND, TRANSACTION_NOT_SETTLED, TRANSACTION_ALREADY_DISPUTED, TRANSACTION_OUTSIDE_ELIGIBILITY_WINDOW) mean there is no disputable transaction behind one or more selected ids for that specific reason; MEMBER_INELIGIBLE means the member is not allowed to dispute at all right now, independent of the selection.