feat: ReAct to fallback if "final answer" part is absent

What does this merge request do and why?

Fallback if "final answer" part is absent

If response doesn't follow expected format, i.e. markers such as "Final answer", currently it would return None.

This behavior is different to monolith's ChainOfThoughtParser, which would fallback to treat the answer as a final answer.

Issue: gitlab-org/gitlab#480563 (closed)

How to set up and validate locally

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.