In entitlements seat manager, it is valid to assign one user to multiple active seats at once. In the GET /seats endpoint, we don't group seats by user, which results in multiple entries containing the same user info. We would like to restrict users to only being in one active seat at a time:

• update POST /seats to check if the desired user has an active seat already, if so return an error with the subscription id of the active seat
• update unit/integration tests to cover this case

slack thread for full context: https://redhat-internal.slack.com/archives/C04MZT67Y9M/p1696268359739669