This commit refactors the AddFinalIntention action for improved behavior and adds comprehensive tests.

Key changes:
- The `isAvailable` method is now more context-aware, only enabling the
  intention when the caret is on a variable (parameter, local variable)
  or within a method scope suitable for adding 'final' modifiers.
- The `invoke` method has been rewritten to:
  - Prioritize making a specific variable final if the caret is on it.
  - Fall back to making all eligible parameters and local variables final
    if the caret is within a method but not on a specific variable.
  - Avoid adding the 'final' modifier if it's already present on an element.
- A comprehensive test suite (`AddFinalIntentionTest.java`) has been added,
  utilizing `LightJavaCodeInsightFixtureTestCase`. The tests cover:
  - Adding 'final' to single parameters and local variables.
  - Adding 'final' to all applicable elements in a method scope.
  - Ensuring no changes occur for already 'final' elements.
  - Verifying the intention is not available in irrelevant contexts
    (e.g., class declarations, import statements).
  - Basic checks for field variables (though full field support might be a future enhancement).

These changes make the intention more robust, predictable, and maintainable.