Backend: Improve spec coverage for agent config update

MR: Pending

Description

The following discussion from !137751 (merged) should be addressed:

• @vtak started a discussion: (+1 comment)

  Author Notes

  Even if we remove .without_terminated in the specs, the tests still pass because by default the agent that is created does not have any workspace which is "terminated" i.e. actual_state == desired_state == "Terminated".