docs: add appAccountToken UUID format requirement

- Update unified-apis.md with UUID format requirement and examples
- Update purchase-flow.md Key iOS Options section
- Update types.md with UUID format note on PurchaseIOS

Refs #128

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: hyochan

docs/docs/api/methods/unified-apis.md

@@ -180,13 +180,15 @@ await requestPurchase({
     apple: {
       sku: productId,
       quantity: 1,
-      appAccountToken: 'user-account-token',
+      appAccountToken: '550e8400-e29b-41d4-a716-446655440000', // Must be UUID format
     },
   },
   type: 'in-app',
 });
 ```
 
+> **⚠️ Important:** The `appAccountToken` must be a valid UUID format. Apple's StoreKit silently returns `null` for non-UUID values. Generate a UUID using `crypto.randomUUID()` or a UUID library. See [OpenIAP Request Types](https://openiap.dev/docs/types/request) for details.
+
 #### iOS with Advanced Commerce Data (iOS 15+)
 
 Use `advancedCommerceData` to pass attribution data (campaign tokens, affiliate IDs) during purchase:
@@ -224,7 +226,7 @@ await requestPurchase({
     - **Android**: `skus` (string[]) - Array of product IDs to purchase
     - **Cross-platform**: Include both `sku` and `skus` for compatibility
     - `quantity?` (number, iOS only): Purchase quantity
-    - `appAccountToken?` (string, iOS only): User identifier for purchase verification
+    - `appAccountToken?` (string, iOS only): User identifier for purchase verification. **Must be a valid UUID format** (e.g., `550e8400-e29b-41d4-a716-446655440000`). If a non-UUID value is provided, Apple will silently return `null` for this field in the purchase response.
     - `advancedCommerceData?` (string, iOS only): Campaign token or attribution data for StoreKit 2's `Product.PurchaseOption.custom` API (iOS 15+)
     - `obfuscatedAccountIdAndroid?` (string, Android only): Obfuscated account ID
     - `obfuscatedProfileIdAndroid?` (string, Android only): Obfuscated profile ID

docs/docs/api/types.md

@@ -116,6 +116,7 @@ export interface PurchaseAndroid extends PurchaseCommon {
 }
 
 export interface PurchaseIOS extends PurchaseCommon {
+  // Note: Must be UUID format when set via requestPurchase, or Apple returns null
   appAccountToken?: string | null;
   environmentIOS?: string | null;
   expirationDateIOS?: number | null;

docs/docs/examples/purchase-flow.md

@@ -93,7 +93,7 @@ Notes:
 
 ### Key iOS Options
 
-- `appAccountToken`: set per user to correlate receipts on your backend
+- `appAccountToken`: set per user to correlate receipts on your backend. **Must be a valid UUID format** (e.g., `550e8400-e29b-41d4-a716-446655440000`). If a non-UUID value is provided, Apple will silently return `null` for this field in the purchase response.
 - `quantity`: purchase quantity for iOS (consumables)
 
 ### Purchase Object Properties