test(amplify_push): add Dart and Android unit tests

[ekjotmultani]

Summary

Adds the Dart and Android (Kotlin) unit tests for the amplify_push package, split into their own PR so the package implementation, the tests, and the example app each review independently.

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

What's here

• Dart unit tests (test/): client lifecycle, flutter API dispatch, permissions, badge, and exception mapping
• Android unit tests (android/src/test/): plugin, background service, events stream handler, notification utils, and Firebase messaging service

Verification

• Dart: 31 tests pass (flutter test). These run automatically in this PR's CI (flutter_vm).
• Android: 33 tests pass (./gradlew :amplify_push:testDebugUnitTest), verified locally.

CI note for the Android tests

In this repo, Android unit tests are example-hosted: aft only generates the amplify_push_android.yaml workflow when both android/src/test/ and example/android/ are present in the package (see generate_workflows_command.dart). Because the tests and the example app are on separate branches, that workflow materializes on main once the package, tests, and example PRs all merge — at which point the 33 Android tests run in CI. Until then they're verified locally (above). If we'd prefer them running in CI pre-merge, the tests branch can be stacked on the example branch instead.

Follow-up: iOS unit tests

The package has no iOS native unit tests yet (only the Flutter template RunnerTests.swift stub). The original push plugin similarly relied on integration/example testing for iOS rather than native XCTest units. Adding native iOS unit tests is tracked as a follow-up.

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 42.98%. Comparing base (9e9a51c) to head (8604cbe).

Additional details and impacted files

@@                   Coverage Diff                    @@
##           feat/amplify-push-v3    #7066      +/-   ##
========================================================
- Coverage                 43.23%   42.98%   -0.25%     
========================================================
  Files                        99      105       +6     
  Lines                      7769     8186     +417     
  Branches                   3401     3586     +185     
========================================================
+ Hits                       3359     3519     +160     
- Misses                     4410     4667     +257     

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