Fast API server: Update session behaviour incorrect as per documentation

**Describe the bug**
Fast API server returns an error when a session with the ID exists instead of updating its state

**To Reproduce**
Steps to reproduce the behavior:
1. Call the /apps/{app_name}/users/{user_id}/sessions/{session_id} for an existing session_id
2. You'll get "Session already exists: "

**Expected behavior**
If the session with that ID exists, its state must be updated.

**Documentation:**

**Actual code:**