Get the funds available for card spend backed by a given Security Account, along with the
balances that amount is derived from.

Open to buy is the amount Lithic authorizes card spend against. It is not a stored balance,
so it is recalculated on every request from the Security Account, the funds held against spend
Lithic has already paid out to the networks on your behalf, and the spend that has not yet been
collected. The accounts that feed the calculation depend on your program setup, so
summary.settled_funds is null outside Commercial Charge.

Supported for Commercial Charge, Dynamic Reserve, and Secured Charge programs. Returns 404
if financial_account_token is not a Security Account you own, or if your program setup does
not use an open to buy calculation.