Make money parameter optional in buyer verification flows

[realmeylisdev]

Summary

Fixes #258

The money parameter in startBuyerVerificationFlow and startCardEntryFlowWithBuyerVerification is now optional (Money?). When buyerAction is "Store" (verifying a saved card on file rather than a charge), the native SDKs on both Android and iOS discard the money value entirely — requiring callers to construct a dummy Money object was unnecessary and non-intuitive.

Problem

Both native SDKs model BuyerAction as a discriminated union:

• Android: BuyerAction.Store() vs BuyerAction.Charge(money)
• iOS: [SQIPBuyerAction storeAction] vs [SQIPBuyerAction chargeActionWithMoney:money]

The Flutter plugin flattened this into String buyerAction + required Money money, losing the parameter-action relationship. This forced callers using the "Store" action to pass a meaningless dummy:

// Before: forced to pass dummy money for Store action
await InAppPayments.startBuyerVerificationFlow(
    buyerAction: "Store",
    money: Money((b) => b..amount = 0..currencyCode = 'USD'), // unused, but required
    ...
);

Reproducing steps

1. Try calling startBuyerVerificationFlow with buyerAction: "Store" and omit money
2. Before this fix: Compile error — money is required
3. After this fix: Compiles and works correctly — money is optional

Changes

Layer	File	Change
Dart	lib/in_app_payments.dart	required Money money → Money? money; added debug assert that money is non-null for "Charge"; money serialized conditionally
Android	CardEntryModule.java	Null-guard moneyMap before getMoney() in both methods
iOS	FSQIPCardEntry.m	Nil-guard moneyMap before _getMoney: in both methods
Docs	doc/reference.md	Updated parameter tables for both methods

After this fix

// Store action — money omitted
await InAppPayments.startBuyerVerificationFlow(
    buyerAction: "Store",
    squareLocationId: locationId,
    contact: contact,
    paymentSourceId: cardOnFileId,
    onBuyerVerificationSuccess: _onSuccess,
    onBuyerVerificationFailure: _onFailure,
);

// Charge action — money still required (enforced by assert in debug mode)
await InAppPayments.startBuyerVerificationFlow(
    buyerAction: "Charge",
    money: Money((b) => b..amount = 100..currencyCode = 'USD'),
    squareLocationId: locationId,
    contact: contact,
    paymentSourceId: nonce,
    onBuyerVerificationSuccess: _onSuccess,
    onBuyerVerificationFailure: _onFailure,
);

Backwards compatibility

This is a non-breaking change. Existing callers passing money continue to compile and work unchanged.

Test plan

• Call startBuyerVerificationFlow with buyerAction: "Store" and no money — should succeed on both Android and iOS
• Call startBuyerVerificationFlow with buyerAction: "Charge" and valid money — should succeed as before
• Call startBuyerVerificationFlow with buyerAction: "Charge" and no money in debug mode — should hit assert
• Same verification for startCardEntryFlowWithBuyerVerification

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