convertToLegacySession

Converts the current Session instance to a legacy AirwallexSession object.

This conversion is primarily required for Local Payment Methods (LPM), as they are not yet supported by the unified Session flow.

Determines which legacy session to create based on Session properties:

• paymentConsentOptions == null → AirwallexPaymentSession (one-off)

• paymentConsentOptions != null && amount == 0 → AirwallexRecurringSession

• paymentConsentOptions != null && amount > 0 → AirwallexRecurringWithIntentSession

Preserves PaymentIntentProvider for Express Checkout scenarios where applicable.

Throws