fix: allow for the use of the --prompt flag with a value outside of prompt mode

[k80bowman]

Work item
#3320

Fixes a bug for the ai:models:call and ai:agents:call commands in heroku-cli-plugin-ai. These commands both have a --prompt flag which allows users to pass prompts to an AI model or agent. The newly-released prompt mode for the CLI also uses a --prompt flag, however, and it was overriding the --prompt flag in the AI plugin. This PR adds a condition such that prompt mode will only begin if the --prompt flag is passed without a value. It also removes --prompt as a help flag.

Testing

1. Check out this branch and run yarn && yarn build
2. If you don't already have the AI plugin installed, install it by running heroku plugins:install @heroku/plugin-ai
3. You'll need to use a test app with an AI model installed (for internal team members, the app test-app-mia should work).
4. Run the command ./bin/run ai:models:call MODEL_NAME -a APP_NAME --prompt "what is 2+2?". Verify that the response is an answer to the question.
5. Run the command ./bin/run ai:models:call MODEL_NAME -a APP_NAME --prompt. Verify that you enter prompt mode.

[justinwilaby]

LGTM - works perfectly.