feat: Allow scenarios to start with When or Then directly

[Kahoulam]

feat: Allow scenarios to start with When or Then directly

Overview

This PR adds when() and then() methods to the BddScenario class, allowing scenarios to start directly with these keywords without requiring given() first. This aligns with standard Gherkin/Cucumber behavior where a scenario can begin with any step keyword.

Changes

• Added when() method to BddScenario: Allows scenarios to start directly with a When step.
• Added then() method to BddScenario: Allows scenarios to start directly with a Then step.
• Refactored BddScenario to separate file: Extracted BddScenario from bdd_base.dart into bdd_scenario.dart as a part file, following the same pattern as BddFeature and BddBackground.
• Added comprehensive doc comments: Added complete API documentation to bdd_scenario.dart with examples for all methods (given, note, when, then).
• Added tests: Created test/src/core/bdd_scenario_test.dart to verify the new functionality works correctly.

Usage Example

// Starting with When (without Given)
Bdd(feature)
  .scenario('calculate sum')
  .when('I add 2 and 3')
  .then('the result should be 5');

// Starting with Then (without Given or When)
Bdd(feature)
  .scenario('result occurs')
  .then('success message appears');

// Traditional flow (still supported)
Bdd(feature)
  .scenario('login')
  .given('user is registered')
  .when('user logs in')
  .then('access granted');

Closes #14