test(waiter): enhance test coverage and fix incorrect expectations

WHAT:
- Added negative test cases for line range validation
- Fixed incorrect test expectations for invalid prompt handling
- Added tests for the new fluent interface API
- Improved assertion messages for better failure diagnostics
- Updated tests to use modern Python idioms

WHY:
- To ensure robust handling of edge cases
- To validate the new fluent interface and PaneContentWaiter class
- To make test failures more informative and easier to debug
- To increase overall test coverage for better code quality

Author: tony