Handle edge case search queries better

We should do some preprocessing of search_query here to prevent errors in edge cases, e.g. where the query ends with an operator like & or |.

Words separated by only a space and no operator also have no effect currently, we could modify these such that & is inserted automatically.

List of valid operations I am aware of:

&
|
<->
!

The syntax also allows combinations like &! and groupings via parantheses.

https://github.com/LAION-AI/Open-Assistant/blob/main/backend/oasst_backend/prompt_repository.py#L1023