chore(ios): fix Swift 6 warnings and deprecated API usage (#3148)

## Summary

- Fix Swift 6 compatibility warnings to prepare for Swift 6 migration
- Suppress unused result warning for better code cleanliness

Closes #3146

## Changes

### RnIapHelper.swift
- Add explicit `@unchecked Sendable` conformance to `OpenIapException`
class

### HybridRnIap.swift
- Capture `products` as local constant before `MainActor.run` closure to
avoid Swift 6 concurrency warning
- Add `_ =` to suppress unused result warning for `finishTransaction`

## Notes

The deprecated API warnings (`validateReceiptIOS` and
`requestPurchaseOnPromotedProductIOS`) are from the OpenIAP library
itself and will be resolved when OpenIAP is updated with new APIs.

## Test plan

- [x] `yarn typecheck` passes
- [x] `yarn lint` passes

🤖 Generated with [Claude Code](https://claude.ai/code)

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

* **Refactor**
* Improved iOS in-app purchase internals to reduce concurrency issues
and streamline transaction handling.
* **Bug Fixes**
* Reduced risk of transaction-related errors and unexpected behavior,
improving reliability of purchases and completion flows.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: hyochan

ios/HybridRnIap.swift

@@ -132,7 +132,7 @@ class HybridRnIap: HybridRnIapSpec {
                 products.append(product)
                 seenIds.insert(product.id)
             }
-            await MainActor.run {
+            await MainActor.run { [products] in
                 products.forEach { self.productTypeBySku[$0.id] = $0.type.lowercased() }
             }
             RnIapLog.result(
@@ -327,7 +327,7 @@ class HybridRnIap: HybridRnIapSpec {
                 let sanitizedPayload = RnIapHelper.sanitizeDictionary(purchasePayload)
                 RnIapLog.payload("finishTransaction.nativePayload", sanitizedPayload)
                 let purchaseInput = try OpenIapSerialization.purchaseInput(from: purchasePayload)
-                try await OpenIapModule.shared.finishTransaction(purchase: purchaseInput, isConsumable: nil)
+                _ = try await OpenIapModule.shared.finishTransaction(purchase: purchaseInput, isConsumable: nil)
                 RnIapLog.result("finishTransaction", true)
                 await MainActor.run {
                     self.purchasePayloadById.removeValue(forKey: iosParams.transactionId)

ios/RnIapHelper.swift

@@ -5,7 +5,7 @@ import OpenIAP
 /// Similar to Android's OpenIapException, this wraps errors with JSON-serialized messages.
 /// Uses NSError for better compatibility with Objective-C bridging in Nitro.
 @available(iOS 15.0, *)
-class OpenIapException: NSError {
+class OpenIapException: NSError, @unchecked Sendable {
     static let domain = "com.margelo.nitro.rniap"
 
     convenience init(_ json: String) {