openapi: fix GET /api/hub/labels response to the label-catalog shape

GET /api/hub/labels returns the catalog of available labels you can filter by,
which the Cloud runtime serves as {labels: HubLabelInfo[]} (slug name,
display_name, and a type category: tag/model/custom_node).

The spec had this operation returning a bare array of HubLabel ({id, name,
color}) — that schema models the label chips attached to a published workflow
(HubWorkflow.labels), a different object. The catalog schema (HubLabelInfo)
already existed but was unreferenced.

Repoints the 200 response to a new HubLabelListResponse wrapper over the
existing HubLabelInfo. HubLabel is unchanged and still used by
HubWorkflow.labels. Endpoint remains x-runtime: [cloud].

openapi.yaml

@@ -3578,10 +3578,7 @@ paths:
           content:
             application/json:
               schema:
-                type: array
+                $ref: "#/components/schemas/HubLabelListResponse"
-                items:
-                  $ref: "#/components/schemas/HubLabel"
-
         '400':
           description: Bad request (e.g. invalid type parameter)
           content:
@@ -10455,6 +10452,19 @@ components:
           - custom_node
           description: Label category.
 
+    HubLabelListResponse:
+      type: object
+      x-runtime: [cloud]
+      description: '[cloud-only] Response wrapper for the available Hub label catalog.'
+      required:
+      - labels
+      properties:
+        labels:
+          type: array
+          items:
+            $ref: '#/components/schemas/HubLabelInfo'
+          description: Available labels, optionally filtered by type.
+
     HubProfileSummary:
       type: object
       x-runtime: [cloud]