Bug: Fix prompt length computation

[Timoeller]

Problem

The current prompt for the Answergenerator can exceed the allowed length. The prompt needs to account for the number of generated tokens as well since "The token count of your prompt plus max_tokens cannot exceed the model's context length.".

Proposed Changes:

Subtract max_tokens from leftover_token_len

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added tests that demonstrate the correct behavior of the change
• I've used the conventional commit convention for my PR title
• I documented my code
• I ran pre-commit hooks and fixed any issue

[ZanSara]

LGTM 👍