refactor action handler execution

[yceruto]

This pull request introduces a refactor of the action handler execution in the form flow system, along with other minor improvements. The refactor simplifies the handling logic by delegating execution responsibilities directly to the ActionButton class, removing redundant methods, and improving code clarity.

Action Handler Refactor:

• src/Form/Flow/ActionButton.php: Added isHandled and handle methods to execute the action handler directly within the ActionButton class, marking the button as handled afterward.
• src/Form/Flow/FormFlow.php: Removed the handleAction method and updated logic to use the handle method of ActionButton for action execution. [1] [2]
• src/Form/Flow/FormFlowInterface.php: Removed the handleAction method declaration as it is no longer needed.

Code Enhancements:

• src/Form/Flow/FormFlowBuilder.php: Improved the getInitialStep method to handle cases where no initial data is provided, ensuring a default step is returned.
• src/Form/Flow/ActionButton.php: Introduced a new property handled to track whether the handler has been executed.

Documentation Updates:

• CHANGELOG.md: Added a changelog documenting breaking changes, fixes, and additions, including the refactor of action handler execution.