Refactor test_generation.py: Parameterize text model tests and add edge case

- Used @pytest.mark.parametrize to simplify and reuse dialog tests in TestTextModelInference.
- Added a new test case for handling empty dialog inputs to improve edge case coverage.
- Maintained the vision model tests in skipped state with @unittest.skip and @pytest.mark.skip.
- Improved overall readability and modularity of the test file.

Update test_generation.py