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/config/api_update_config.py

@@ -11,6 +11,7 @@ router = APIRouter()
     "/config/deployment-limit",
     tags=["Configuration"],
     responses={
+        400: {"description": "Bad Request (e.g. malformed JSON)"},
         401: {"description": "Could not validate credentials"},
         403: {"description": "Admin access required"},
         422: {"description": "Validation error"},
@@ -29,6 +30,7 @@ async def api_update_deployment_limit(
     "/config/global-mutation-interval",
     tags=["Configuration"],
     responses={
+        400: {"description": "Bad Request (e.g. malformed JSON)"},
         401: {"description": "Could not validate credentials"},
         403: {"description": "Admin access required"},
         422: {"description": "Validation error"},