feat(amplify_push): add example app

[ekjotmultani]

Summary

Adds a runnable example app for the amplify_push package so the push APIs can be exercised on a real device.

Stacked on #6985. This PR targets the feat/amplify-push-v3 branch (the package PR), so the diff shows only the example app. Once #6985 merges, this will be rebased onto main.

What the example does

• Requests notification permission and shows current permission status
• Displays the live device token from onTokenReceived
• Shows foreground, opened, and launch notifications with an on-screen event log (no debugger needed)
• iOS badge handling

Running it

cd packages/amplify_push/example
# Android: drop google-services.json in android/app/ (a placeholder is included)
# iOS: configure APNs in Xcode
flutter run

Send a test push via the Firebase Console (Messaging > Send test message) or curl for Android, or an APNs payload for iOS. See example/README.md for exact commands.

Verification

• iOS builds with --no-codesign; AmplifyUtilsNotifications pod resolves
• Example analyzes clean

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.23%. Comparing base (9e9a51c) to head (4752904).

Additional details and impacted files

@@                  Coverage Diff                  @@
##           feat/amplify-push-v3    #7065   +/-   ##
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  Coverage                 43.23%   43.23%           
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  Files                        99       99           
  Lines                      7769     7769           
  Branches                   3401     3401           
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  Hits                       3359     3359           
  Misses                     4410     4410           

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