Backend: Improve spec coverage for agent config update
MR: Pending
<!--
The first line of the MR must be one of the following:
1. `MR: Pending`
2. `MR: <MR link with trailing +>`,
and the first description line of the MR should be `Issue: <Issue link with trailing +>`
3. `MR: No MR`
For more context, see:
https://about.gitlab.com/handbook/engineering/development/dev/create/ide/index.html#1-to-1-relationship-of-issues-to-mrs
-->
<!--
The following sections should be filled out as part of the refinement process before the issue is prioritized.
For more context, see:
https://about.gitlab.com/handbook/engineering/development/dev/create/ide/#2-pre-iteration-planning-meeting
-->
## Description
The following discussion from !137751 should be addressed:
- [ ] @vtak started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/137751#note_1663562969): (+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".
<!-- Replace with other type, e.g. bug or maintenance, if appropriate -->
<!-- Replace with other subtype if appropriate -->
<!-- By default, all issues start in the unprioritized status. See https://about.gitlab.com/handbook/engineering/development/dev/create/ide/#-remote-development-planning-process -->
<!-- For simplicity and to avoid triage bot warnings about missing workflow labels, we will default to issues starting at the refinement phase -->
issue