fix(flipcash): treat an unset bill_exchange_data_timeout as absent

[bmc08gt]

bill_exchange_data_timeout is a google.protobuf.Duration, so an unset field still deserializes to the default instance. UserFlagsMapper read it unconditionally and turned that into 0.seconds, even though the domain field is Duration? and UserFlags.Default uses null. A consumer could not tell "no timeout configured" from "timeout of zero".

This guards the mapping on hasBillExchangeDataTimeout(), which is how every other optional message-typed field in this package is read — UserProfileMapper's profilePicture and joinTs, ChatMetadataMapper's lastMessage.

Why the distinction matters

VerifiedState.exchangeDataFor treats the two values as opposites:

val timeout = billExchangeDataTimeout ?: DefaultBillExchangeDataTimeout // 15.minutes
if (timeout <= Duration.ZERO) return null

So for any user whose flags omitted the field, GiveBillTransactor.start discarded the VerifiedState it already held — however fresh the rate was — and fell into its "rate expired" branch, refetching a verified state on every bill presentation. When that refetch failed (offline, or no rate for the currency/mint pair) the give flow returned ExchangeRateExpiredException while holding a usable rate. With null the held state gets the intended 15 minute window.

A server that deliberately sends 0s still disables verified exchange data, unchanged.

Consumers

No changes needed. Field.BillExchangeDataTimeout already encodes null as -1L and formats it as "None", CachedFlags round-trips it as a nullable Long, and TransactionController's gift-card path never passed the flag — it relies on the same null default.

The one visible difference is the debug flags screen, which now reads "None" rather than "0s" when the server omits the field.