fix: [SDK-5161] support shared template notification actions

[fadi-george]

One Line Summary

Adds shared iOS template action-button icons to Flutter demos and fixes Android grouped-notification click serialization.

Motivation

Template action icons must be bundled in both the app and notification service extension. During cross-wrapper verification, grouped summary clicks also exposed that Flutter passed native notification objects over the method channel instead of serializable maps.

Scope

• Add template-bookmark-icon.png and template-share-icon.png to five iOS demos.
• Include both icons in the Runner and OneSignalNotificationServiceExtension resource phases.
• Document dashboard icon names and iOS verification steps.
• Recursively serialize Android grouped notifications as maps.
• Accept mapped grouped-notification lists while retaining legacy JSON-string parsing.
• Add grouped-notification parsing regression coverage.

Testing

• flutter test test/notification_test.dart (40 tests passed)
• examples/demo/android/gradlew :onesignal_flutter:compileDebugJavaWithJavac
• Validated all modified Xcode project files with plutil -lint.
• Verified Runner and NSE resource membership in all five demos.
• Physical iOS 15+ icon rendering remains to be verified.

Affected code checklist

• Android bridge serialization
• Dart notification model and tests
• iOS demo resources and Xcode projects
• Demo documentation

Checklist confirmations

• No public API fields were removed or renamed.
• Existing grouped-notification JSON-string parsing remains supported.
• Relevant automated tests and builds pass.
• Documentation describes icon configuration without the .png suffix.

[abdulraqeeb33]

The Android grouped children now serialize as maps, and Dart still accepts the legacy JSON-string path. Demo icons land in Runner and NSE. Approving.