Add rxdart_flutter tests to GHA

[hoc081098]

This pull request includes several changes aimed at updating test configurations and dependencies, as well as refactoring imports and adding a new utility class. The most important changes include renaming and adding test scripts, removing a dependency, refactoring imports, and adding a new ValueSubject class.

Test configuration updates:

• .github/workflows/rxdart-test.yml: Renamed the test job to Run rxdart tests and added a new job to run rxdart_flutter tests.
• melos.yaml: Added a new script test-rxdart-flutter to run tests for the rxdart_flutter package.

Dependency updates:

• packages/rxdart_flutter/pubspec.yaml: Removed the rxdart_ext dependency.

Import refactoring:

• packages/rxdart_flutter/test/src/not_replay_value_stream_builder_test.dart, packages/rxdart_flutter/test/src/not_replay_value_stream_consumer_test.dart, packages/rxdart_flutter/test/src/not_replay_value_stream_listener_test.dart: Refactored imports to replace rxdart_ext with a local import of rxdart_ext/value_subject.dart. [1] [2] [3]

New utility class:

• packages/rxdart_flutter/test/src/rxdart_ext/value_subject.dart: Added the ValueSubject class, which is a special StreamController that captures the latest item added to the controller, along with its associated stream and event handling logic.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.40%. Comparing base (81b909f) to head (11e960c).
Report is 20 commits behind head on master.

Additional details and impacted files

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  Coverage   93.40%   93.40%           
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  Files          78       78           
  Lines        2456     2456           
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  Hits         2294     2294           
  Misses        162      162