[Dart] Improve test coverage

[Fried-man]

🤔 What's changed?

The dart solution now has full line coverage via unit test by:

• excluding generated files from coverage reporting
• adding more tests

Also, the dart CI now enforces full line coverage.

⚡️ What's your motivation?

Outside of best practices, the low line coverage was brought up in this issue.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)

♻️ Anything particular you want feedback on?

N/A

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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