Raise an exception in MKRL and Chat Output Parsers if parsing text which contains both an action and a final answer

[nathan-az]

Raises exception if OutputParsers receive a response with both a valid action and a final answer

Currently, if an OutputParser receives a response which includes both an action and a final answer, they return a FinalAnswer object. This allows the parser to accept responses which propose an action and hallucinate an answer without the action being parsed or taken by the agent.

This PR changes the logic to:

1. store a variable checking whether a response contains the FINAL_ANSWER_ACTION (this is the easier condition to check).
2. store a variable checking whether the response contains a valid action
3. if both are present, raise a new exception stating that both are present
4. if an action is present, return an AgentAction
5. if an answer is present, return an AgentAnswer
6. if neither is present, raise the relevant exception based around the action format (these have been kept consistent with the prior exception messages)

Disclaimer:

• Existing mock data included strings which did include an action and an answer. This might indicate that prioritising returning AgentAnswer was always correct, and I am patching out desired behaviour? @hwchase17 to advice. Curious if there are allowed cases where this is not hallucinating, and we do want the LLM to output an action which isn't taken.
• I have not passed send_to_llm through this new exception

Fixes #5601

Who can review?

Community members can review the PR once tests pass. Tag maintainers/contributors who might be interested:

@hwchase17 - project lead
@vowelparrot

[hwchase17]

this lgtm!