fix(api): document 400 on topology read endpoints for schemathesis contract

DECNET's app-level RequestValidationError handler remaps structural
422→400, including query/path constraint violations (limit bounds,
the next-subnet base pattern, etc.).  Schemathesis fuzzing will drive
those code paths and fail response_schema_conformance unless 400 is
declared in responses={}.  Adds the entry to every phase-3 read route.

decnet/web/router/topology/api_catalog.py

@@ -30,6 +30,7 @@ router = APIRouter()
     tags=["MazeNET Topologies"],
     response_model=ServiceCatalogResponse,
     responses={
+        400: {"description": "Malformed query parameters"},
         401: {"description": "Missing or invalid credentials"},
         403: {"description": "Insufficient permissions"},
     },
@@ -70,6 +71,7 @@ async def api_next_subnet(
     tags=["MazeNET Topologies"],
     response_model=NextIPResponse,
     responses={
+        400: {"description": "Malformed path parameters"},
         401: {"description": "Missing or invalid credentials"},
         403: {"description": "Insufficient permissions"},
         404: {"description": "Topology or LAN not found"},

decnet/web/router/topology/api_get_topology.py

@@ -23,6 +23,7 @@ router = APIRouter()
     tags=["MazeNET Topologies"],
     response_model=TopologyDetail,
     responses={
+        400: {"description": "Malformed path parameters"},
         401: {"description": "Missing or invalid credentials"},
         403: {"description": "Insufficient permissions"},
         404: {"description": "Topology not found"},
@@ -49,6 +50,7 @@ async def api_get_topology(
     tags=["MazeNET Topologies"],
     response_model=list[TopologyStatusEventRow],
     responses={
+        400: {"description": "Malformed query parameters"},
         401: {"description": "Missing or invalid credentials"},
         403: {"description": "Insufficient permissions"},
         404: {"description": "Topology not found"},

decnet/web/router/topology/api_list_topologies.py

@@ -17,6 +17,7 @@ router = APIRouter()
     tags=["MazeNET Topologies"],
     response_model=TopologyListResponse,
     responses={
+        400: {"description": "Malformed query parameters"},
         401: {"description": "Missing or invalid credentials"},
         403: {"description": "Insufficient permissions"},
     },