test(amplify_push): add Dart and Android unit tests

[ekjotmultani]

Unit tests for amplify_push, split out for review scoping. Stacked on #6985 (targets feat/amplify-push-v3), so the diff shows only the test files; rebased onto main once #6985 merges.

• Dart unit tests (client, flutter API, exception mapping, message parsing)
• Android (Kotlin) unit tests (plugin, background service, stream handler, notification utils, Firebase messaging)

Verified locally: 31 Dart tests pass; analyzer clean. (Android unit tests run in CI on main once the example + tests coexist.)

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 42.83%. Comparing base (48cb67a) to head (849c1a6).

Additional details and impacted files

@@                   Coverage Diff                    @@
##           feat/amplify-push-v3    #7078      +/-   ##
========================================================
- Coverage                 43.23%   42.83%   -0.40%     
========================================================
  Files                        99      105       +6     
  Lines                      7769     8252     +483     
  Branches                   3401     3602     +201     
========================================================
+ Hits                       3359     3535     +176     
- Misses                     4410     4717     +307     

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