Fix pagination test response parsing

The pagination tests were incorrectly parsing the API response. The API
returns data wrapped in a {"data": [...]} structure, but the tests were
expecting a direct array. This caused tests to fail silently when
json.Decode couldn't match the response structure.

Changed response parsing to correctly extract the "data" field before
asserting on array length.

cmd/server/tests/media_item_isbn_test.go

@@ -334,11 +334,12 @@ func TestMediaItemsPagination(t *testing.T) {
 
 		assert.Equal(t, http.StatusOK, resp.StatusCode)
 
-		var response []map[string]interface{}
+		var response map[string]interface{}
 		json.NewDecoder(resp.Body).Decode(&response)
+		data := response["data"].([]interface{})
 
 		// Should get 2 items
-		assert.Equal(t, 2, len(response))
+		assert.Equal(t, 2, len(data))
 	})
 
 	t.Run("Pagination with offset", func(t *testing.T) {
@@ -352,11 +353,12 @@ func TestMediaItemsPagination(t *testing.T) {
 
 		assert.Equal(t, http.StatusOK, resp.StatusCode)
 
-		var response []map[string]interface{}
+		var response map[string]interface{}
 		json.NewDecoder(resp.Body).Decode(&response)
+		data := response["data"].([]interface{})
 
 		// Should get 2 items starting from offset 2
-		assert.Equal(t, 2, len(response))
+		assert.Equal(t, 2, len(data))
 	})
 
 	t.Run("Negative limit should fail", func(t *testing.T) {