[BUG] Weird Memory Retention from Sherpa

Describe the bug
In the Slack Channel, Sherpa responded to a question with an irrelevant search:

After some investigation, it seems this query was also used to answer another question earlier:

So for some reason, the memory is retained from the last question to this question. However, this seems to happen rarely so more investigation is necessary.

To Reproduce
Unclear

Expected behavior
Sherpa should answer the second question by searching with a relevant query

It seems this line:

sherpa/src/sherpa_ai/agents/qa_agent.py

Line 36 in 1547d26

belief: Belief = Belief(),

causes all QA agents with default beliefs to share the same belief. I think this may be the cause of the issue. There is already a fix implemented in the above PR mentioned in this issue.