Fix group avatar API

Group avatar API endpoint was not working for subgroups like 'g1/g1.1'.
This commit fixes this issue by properly parsing the URL.

Changelog: fixed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63934

lib/api/group_avatar.rb

@@ -6,13 +6,13 @@ module API
 
     feature_category :subgroups
 
-    resource :groups do
+    params do
+      requires :id, type: String, desc: 'The ID of a group'
+    end
+    resource :groups, requirements: API::NAMESPACE_OR_PROJECT_REQUIREMENTS do
       desc 'Download the group avatar' do
         detail 'This feature was introduced in GitLab 14.0'
       end
-      params do
-        requires :id, type: String, desc: 'The group id'
-      end
       get ':id/avatar' do
         present_carrierwave_file!(user_group.avatar)
       end

spec/requests/api/group_avatar_spec.rb

@@ -4,7 +4,7 @@ require 'spec_helper'
 
 RSpec.describe API::GroupAvatar do
   def avatar_path(group)
-    "/groups/#{group.id}/avatar"
+    "/groups/#{ERB::Util.url_encode(group.full_path)}/avatar"
   end
 
   describe 'GET /groups/:id/avatar' do
@@ -26,6 +26,16 @@ RSpec.describe API::GroupAvatar do
           expect(response).to have_gitlab_http_status(:not_found)
         end
       end
+
+      context 'when the group is a subgroup' do
+        it 'returns :ok' do
+          group = create(:group, :nested, :public, :with_avatar, name: 'g1.1')
+
+          get api(avatar_path(group))
+
+          expect(response).to have_gitlab_http_status(:ok)
+        end
+      end
     end
 
     context 'when the group is private' do