refactor(tests): improve test structure and clarity for multiline commit feature

Refactored test files to improve maintainability and clarity:

**test_commit_command.py:**
- Created reusable mock_prompt fixture to eliminate duplicated mocking code
- Updated all tests to use the new fixture pattern with _handle_questionary_prompt
- Improved test for message length limit to properly test both success and failure cases
- Simplified test logic by removing unnecessary variable assignments

**test_commit_multiline.py:**
- Restructured tests into focused test classes by functionality
- Simplified test logic to directly test _handle_multiline_question function
- Added parameterized tests for optional field detection
- Improved test coverage for filter errors with proper retry behavior
- Removed redundant implementation-level tests in favor of behavior tests
- Enhanced test documentation and clarity

Author: anasalaqeel