feat(google): support one-time purchase discount offers (#51)

BREAKING CHANGE: `oneTimePurchaseOfferDetailsAndroid` type changed from
single object to array to support multiple discount offers.

Migration:
- Before: `product.oneTimePurchaseOfferDetailsAndroid?.formattedPrice`
- After:
`product.oneTimePurchaseOfferDetailsAndroid?.firstOrNull()?.formattedPrice`

Changes:
- Add discount-related types: DiscountDisplayInfoAndroid,
DiscountAmountAndroid,
ValidTimeWindowAndroid, LimitedQuantityInfoAndroid,
PreorderDetailsAndroid,
  RentalDetailsAndroid
- Update BillingConverters to use oneTimePurchaseOfferDetailsList API
  (Google Play Billing Library 7.0+)
- Add discount display support in Example app (ProductCard, Modals)
- Add discount feature documentation
- Fix verifyPurchaseWithIapkit return type usage in Example screens

Related issue: hyochan/react-native-iap#3102

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Android: Added discount support for one-time purchases
(percentage/amount), multiple offers per product, validity windows,
limited-quantity and rental metadata.
* UI: Shows original price when discounted, highlights discounted price
and displays discount badges and per-offer details.

* **Documentation**
* New comprehensive "Discounts (Android)" guide with multi-language
examples, UI patterns, and best practices.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: hyochan

packages/apple/Sources/Models/Types.swift

@@ -324,6 +324,26 @@ public struct AppTransaction: Codable {
     public var signedDate: Double
 }
 
+/// Discount amount details for one-time purchase offers (Android)
+/// Available in Google Play Billing Library 7.0+
+public struct DiscountAmountAndroid: Codable {
+    /// Discount amount in micro-units (1,000,000 = 1 unit of currency)
+    public var discountAmountMicros: String
+    /// Formatted discount amount with currency sign (e.g., "$4.99")
+    public var formattedDiscountAmount: String
+}
+
+/// Discount display information for one-time purchase offers (Android)
+/// Available in Google Play Billing Library 7.0+
+public struct DiscountDisplayInfoAndroid: Codable {
+    /// Absolute discount amount details
+    /// Only returned for fixed amount discounts
+    public var discountAmount: DiscountAmountAndroid?
+    /// Percentage discount (e.g., 33 for 33% off)
+    /// Only returned for percentage-based discounts
+    public var percentageDiscount: Int?
+}
+
 public struct DiscountIOS: Codable {
     public var identifier: String
     public var localizedPrice: String?
@@ -376,6 +396,15 @@ public enum FetchProductsResult {
     case subscriptions([ProductSubscription]?)
 }
 
+/// Limited quantity information for one-time purchase offers (Android)
+/// Available in Google Play Billing Library 7.0+
+public struct LimitedQuantityInfoAndroid: Codable {
+    /// Maximum quantity a user can purchase
+    public var maximumQuantity: Int
+    /// Remaining quantity the user can still purchase
+    public var remainingQuantity: Int
+}
+
 /// Pre-order details for one-time purchase products (Android)
 /// Available in Google Play Billing Library 8.1.0+
 public struct PreorderDetailsAndroid: Codable {
@@ -408,21 +437,43 @@ public struct ProductAndroid: Codable, ProductCommon {
     public var displayPrice: String
     public var id: String
     public var nameAndroid: String
-    public var oneTimePurchaseOfferDetailsAndroid: ProductAndroidOneTimePurchaseOfferDetail?
+    /// One-time purchase offer details including discounts (Android)
+    /// Returns all eligible offers. Available in Google Play Billing Library 7.0+
+    public var oneTimePurchaseOfferDetailsAndroid: [ProductAndroidOneTimePurchaseOfferDetail]?
     public var platform: IapPlatform = .android
     public var price: Double?
     public var subscriptionOfferDetailsAndroid: [ProductSubscriptionAndroidOfferDetails]?
     public var title: String
     public var type: ProductType = .inApp
 }
 
+/// One-time purchase offer details (Android)
+/// Available in Google Play Billing Library 7.0+
 public struct ProductAndroidOneTimePurchaseOfferDetail: Codable {
+    /// Discount display information
+    /// Only available for discounted offers
+    public var discountDisplayInfo: DiscountDisplayInfoAndroid?
     public var formattedPrice: String
-    /// Pre-order details for products available for pre-order (Android)
+    /// Full (non-discounted) price in micro-units
+    /// Only available for discounted offers
+    public var fullPriceMicros: String?
+    /// Limited quantity information
+    public var limitedQuantityInfo: LimitedQuantityInfoAndroid?
+    /// Offer ID
+    public var offerId: String?
+    /// List of offer tags
+    public var offerTags: [String]
+    /// Offer token for use in BillingFlowParams when purchasing
+    public var offerToken: String
+    /// Pre-order details for products available for pre-order
     /// Available in Google Play Billing Library 8.1.0+
     public var preorderDetailsAndroid: PreorderDetailsAndroid?
     public var priceAmountMicros: String
     public var priceCurrencyCode: String
+    /// Rental details for rental offers
+    public var rentalDetailsAndroid: RentalDetailsAndroid?
+    /// Valid time window for the offer
+    public var validTimeWindow: ValidTimeWindowAndroid?
 }
 
 public struct ProductIOS: Codable, ProductCommon {
@@ -451,7 +502,9 @@ public struct ProductSubscriptionAndroid: Codable, ProductCommon {
     public var displayPrice: String
     public var id: String
     public var nameAndroid: String
-    public var oneTimePurchaseOfferDetailsAndroid: ProductAndroidOneTimePurchaseOfferDetail?
+    /// One-time purchase offer details including discounts (Android)
+    /// Returns all eligible offers. Available in Google Play Billing Library 7.0+
+    public var oneTimePurchaseOfferDetailsAndroid: [ProductAndroidOneTimePurchaseOfferDetail]?
     public var platform: IapPlatform = .android
     public var price: Double?
     public var subscriptionOfferDetailsAndroid: [ProductSubscriptionAndroidOfferDetails]
@@ -609,6 +662,16 @@ public struct RenewalInfoIOS: Codable {
     public var willAutoRenew: Bool
 }
 
+/// Rental details for one-time purchase products that can be rented (Android)
+/// Available in Google Play Billing Library 7.0+
+public struct RentalDetailsAndroid: Codable {
+    /// Rental expiration period in ISO 8601 format
+    /// Time after rental period ends when user can still extend
+    public var rentalExpirationPeriod: String?
+    /// Rental period in ISO 8601 format (e.g., P7D for 7 days)
+    public var rentalPeriod: String
+}
+
 public enum RequestPurchaseResult {
     case purchase(Purchase?)
     case purchases([Purchase]?)
@@ -658,6 +721,15 @@ public struct UserChoiceBillingDetails: Codable {
     public var products: [String]
 }
 
+/// Valid time window for when an offer is available (Android)
+/// Available in Google Play Billing Library 7.0+
+public struct ValidTimeWindowAndroid: Codable {
+    /// End time in milliseconds since epoch
+    public var endTimeMillis: String
+    /// Start time in milliseconds since epoch
+    public var startTimeMillis: String
+}
+
 public struct VerifyPurchaseResultAndroid: Codable {
     public var autoRenewing: Bool
     public var betaProduct: Bool

packages/docs/src/pages/docs.tsx

@@ -10,6 +10,7 @@ import Events from './docs/events';
 import Errors from './docs/errors';
 import Purchase from './docs/features/purchase';
 import SubscriptionFeature from './docs/features/subscription';
+import Discount from './docs/features/discount';
 import OfferCodeRedemption from './docs/features/offer-code-redemption';
 import ExternalPurchase from './docs/features/external-purchase';
 import SubscriptionUpgradeDowngrade from './docs/features/subscription-upgrade-downgrade';
@@ -170,6 +171,15 @@ function Docs() {
                 Subscription
               </NavLink>
             </li>
+            <li>
+              <NavLink
+                to="/docs/features/discount"
+                className={({ isActive }) => (isActive ? 'active' : '')}
+                onClick={closeSidebar}
+              >
+                Discounts (Android)
+              </NavLink>
+            </li>
             <li>
               <NavLink
                 to="/docs/features/offer-code-redemption"
@@ -245,6 +255,7 @@ function Docs() {
             path="features/subscription"
             element={<SubscriptionFeature />}
           />
+          <Route path="features/discount" element={<Discount />} />
           <Route
             path="features/offer-code-redemption"
             element={<OfferCodeRedemption />}