Add Dart 3.8.3 to CI matrix

[Copilot]

Extends CI coverage to test against both stable and Dart 3.8.3.

Changes

• Added matrix strategy to all workflow jobs (6 test jobs, 2 code quality jobs)
• Each job now runs twice: once with stable, once with 3.8.3

strategy:
  matrix:
    sdk: ['stable', '3.8.3']
steps:
  - name: Setup Dart
    uses: dart-lang/setup-dart@v1
    with:
      sdk: ${{ matrix.sdk }}

This ensures compatibility with both the latest stable release and the specific 3.8.3 version.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add Dart 3.8.1 in CI matrix</issue_title>
<issue_description>Currently we are only testing with stable Dart. But we should run all CI workflows with Dart 3.8.1 as well</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add Dart 3.8.1 in CI matrix #62

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