Fix(llm-cli): Ensure script exits after displaying help message

[DealsBeam]

The llm-cli script did not exit after displaying the help message when run with the -h or --help flags. This caused the script to continue execution and display an "Invalid model_family" error.

This commit adds an exit 0 command after the display_help function is called, ensuring that the script exits gracefully after printing the help message.

A new test case has been added to python/llm/test/cli/test_cli.py to verify that the fix is working correctly and to prevent future regressions.