resolvedRequiredBillingContactFields

Effective set of billing contact fields, resolving null to the legacy boolean-derived defaults that match Android's pre-existing UI:

• NAME is always included (the cardholder-name row was always shown).

• AirwallexSession.isBillingInformationRequired = true → adds ADDRESS + PHONE.

• AirwallexSession.isEmailRequired = true → adds EMAIL.

Merchants who explicitly call setRequiredBillingContactFields(...) bypass this derivation entirely; an empty set hides the entire billing section including the cardholder-name row.