Align onDeferredPurchaseCompleted param name with iOS (purchaseResult)

[NickSxti]

Summary

Rename the supertype param from `transaction` to `purchaseResult` in `QonversionEventsListener.onDeferredPurchaseCompleted`. Aligns with iOS naming (`RNQonversion.mm` uses `purchaseResult`) and the param name already chosen by the override in react-native-sdk and flutter-sdk.

Without this rename, both react-native-sdk and flutter-sdk emit a Kotlin compiler warning on every build:

```
warning: the corresponding parameter in the supertype 'QonversionEventsListener' is named 'transaction'. This may cause problems when calling this function with named arguments.
```

(See react-native-sdk #439 for the original report.)

Compatibility

• The only call site inside sandwich-sdk (`QonversionSandwich.kt:512`) passes a positional argument, so this is purely a metadata change.
• No named-argument callers exist across the Qonversion repos (verified with `grep -rn 'onDeferredPurchaseCompleted(transaction =' .`).
• Java consumers (cordova-plugin, unity-sdk) do not use Kotlin named arguments and are unaffected.
• Kotlin consumers (react-native-sdk, flutter-sdk) override with `purchaseResult` already, so the rename eliminates their warning instead of introducing a new one.

Follow-up after release

Once a sandwich version containing this change is published, two follow-up PRs will bump the dep:

• react-native-sdk: `android/build.gradle` line 83 - `io.qonversion:sandwich:7.9.0` -> next.
• flutter-sdk: `android/build.gradle` line 54 - same bump.

Single-line change, +1/-1.