fix: resolve schemathesis and live test failures

- Add 403 response to all RBAC-gated endpoints (schemathesis UndefinedStatusCode)
- Add 400 response to all endpoints accepting JSON bodies (malformed input)
- Add required 'title' field to schemathesis.toml for schemathesis 4.15+
- Add xdist_group markers to live tests with module-scoped fixtures to
  prevent xdist from distributing them across workers (fixture isolation)

decnet/web/router/health/api_get_health.py

@@ -18,6 +18,7 @@ _OPTIONAL_SERVICES = {"sniffer_worker"}
     tags=["Observability"],
     responses={
         401: {"description": "Could not validate credentials"},
+        403: {"description": "Insufficient permissions"},
         503: {"model": HealthResponse, "description": "System unhealthy"},
     },
 )