FIX: Load categories with search topic results (#25700)

Add categories to the serialized search results together with the topics
when lazy load categories is enabled. This is necessary in order for the
results to be rendered correctly and display the category information.

spec/requests/api/schemas/json/search_response.json

@@ -74,6 +74,17 @@
             "null"
           ]
         },
+        "extra": {
+          "type": "object",
+          "properties": {
+            "categories": {
+              "type": [
+                "array",
+                "null"
+              ]
+            }
+          }
+        },
         "post_ids": {
           "type": "array",
           "items": {