Add collection ID to `/collection/:id/items` (#41757)

Pretty much does what it says on the tin.

src/metabase/api/collection.clj

@@ -635,6 +635,7 @@
   `post-process-collection-children`. Must respect the order passed in."
   [collection rows]
   (->> (map-indexed (fn [i row] (vary-meta row assoc ::index i)) rows) ;; keep db sort order
+       (map #(assoc % :collection_id (:id collection)))
        (map remove-unwanted-keys)
        (group-by :model)
        (into []

test/metabase/api/collection_test.clj

@@ -647,7 +647,8 @@
                                                                         :moderator_id        user-id
                                                                         :most_recent         true}]
         (is (= (mt/obj->json->obj
-                [{:id                  card-id
+                [{:collection_id       (:id collection)
+                  :id                  card-id
                   :location            nil
                   :name                (:name card)
                   :collection_position nil