fix(api): document missing HTTP status codes on router endpoints

Schemathesis was failing CI on routes that returned status codes not
declared in their OpenAPI responses= dicts. Adds the missing codes
across swarm_updates, swarm_mgmt, swarm, fleet and attackers routers.

Also adds 400 to every POST/PUT/PATCH that accepts a JSON body —
Starlette returns 400 on malformed/non-UTF8 bodies before FastAPI's
422 validation runs, which schemathesis fuzzing trips every time.

No handler logic changed.

decnet/web/router/swarm/api_enroll_host.py

@@ -29,7 +29,11 @@ router = APIRouter()
     response_model=SwarmEnrolledBundle,
     status_code=status.HTTP_201_CREATED,
     tags=["Swarm Hosts"],
-    responses={409: {"description": "A worker with this name is already enrolled"}},
+    responses={
+        400: {"description": "Bad Request (malformed JSON body)"},
+        409: {"description": "A worker with this name is already enrolled"},
+        422: {"description": "Request body validation error"},
+    },
 )
 async def api_enroll_host(
     req: SwarmEnrollRequest,