Executes an action on a transaction request. Supported actions: REJECT — rejects a transaction request that is awaiting funds, before it reaches the blockchain. Only requests in FUNDS_NEEDED or FUNDS_REQUESTED are rejectable; any other status is refused, including a request awaiting an approval decision (reject it through that approval) and one already published or in a final status. Repeating the action on an already rejected request succeeds without further effect. No Idempotency-Key is required: the action is idempotent on the state of the request itself, not on a client-supplied key.