fast_llm: Fix UnboundLocalError, add docstring

[endolith]

Previously, fast_llm was trying to access 'response' in the finally block, which would fail with UnboundLocalError if the chat call raised an exception (because of missing API key, etc.)

UnboundLocalError: cannot access local variable 'response' where it is not associated with a value

Moved the return statement into the try block while keeping state restoration in finally, ensuring proper error propagation while maintaining conversation state.

This way, if there's an API key issue or any other problem, any errors from llm.interpreter.chat() propagate up and can be handled at the appropriate level in the call stack.

Added a detailed docstring to clarify the purpose, behavior, and usage of the fast_llm function.

Describe the changes you have made:

Reference any relevant issues (e.g. "Fixes #000"):

Pre-Submission Checklist (optional but appreciated):

• [ ] I have included relevant documentation updates (stored in /docs)
• [x] I have read docs/CONTRIBUTING.md
• [ ] I have read docs/ROADMAP.md

OS Tests (optional but appreciated):

• [x] Tested on Windows
• [ ] Tested on MacOS
• [ ] Tested on Linux