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_heartbeat.py

@@ -101,8 +101,10 @@ async def _verify_peer_matches_host(
     status_code=204,
     tags=["Swarm Health"],
     responses={
+        400: {"description": "Bad Request (malformed JSON body)"},
         403: {"description": "Peer cert missing, or its fingerprint does not match the host's pinned cert"},
         404: {"description": "host_uuid is not enrolled"},
+        422: {"description": "Request body validation error"},
     },
 )
 async def heartbeat(