Align error code enums to PascalCase (#554)

Standardizes error code enums to PascalCase to stay aligned with OpenIAP
naming and updates mappings, docs, samples, and tests accordingly.

Tests: `dart test` *(fails: flutter_test requires a Flutter host
environment; run `flutter test` locally)*


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

- Refactor
- Breaking change: public ErrorCode members renamed from e-prefixed
lowerCamelCase to PascalCase (e.g., eUserCancelled → UserCancelled).
Update integrations and error handling.
- New Features
- Added many new error codes to broaden platform and billing coverage
(e.g., BillingUnavailable, ProductAlreadyOwned, PurchaseNotAllowed,
NotInitialized, AlreadyInitialized, etc.).
- Documentation
- Updated API docs, guides, migration notes, examples, and mappings to
reflect renamed and added ErrorCode values.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: hyochan

CHANGELOG.md

@@ -467,7 +467,7 @@ This major release redesigns the API to fully comply with the [Open IAP](https:/
 - **iOS**: Now requires iOS 11.0+ with StoreKit 2 support (iOS 15.0+)
 - **Android**: Updated to Billing Client v8.0.0
 - **API Changes**:
-  - Enum naming convention: `E_UNKNOWN` → `eUnknown` (lowerCamelCase)
+  - Enum naming convention: `E_UNKNOWN` → `Unknown` (PascalCase)
   - Channel access: `FlutterInappPurchase.channel` → `FlutterInappPurchase.instance.channel`
   - Unified error handling with standardized error codes
 

docs/docs/api/core-methods.md

@@ -564,10 +564,10 @@ try {
   await FlutterInappPurchase.instance.initConnection();
 } on PurchaseError catch (e) {
   switch (e.code) {
-    case ErrorCode.eAlreadyInitialized:
+    case ErrorCode.AlreadyInitialized:
       print('Already initialized');
       break;
-    case ErrorCode.eNetworkError:
+    case ErrorCode.NetworkError:
       print('Network error - check connection');
       break;
     default:
@@ -645,7 +645,7 @@ Future<void> makePurchase(String sku) async {
 
     // Success handling happens in purchaseUpdated listener
   } on PurchaseError catch (e) {
-    if (e.code == ErrorCode.eUserCancelled) {
+    if (e.code == ErrorCode.UserCancelled) {
       // User cancelled - don't show error
       return;
     }