Add BDD regression test for Forgejo issue 4321: ActorRegistry.add()
fails to extract provider and model from nested actor config.
The test verifies that when type is only at the nested actors map level
(not at top level), provider and model are correctly extracted from the
nested config block.
This scenario is already handled correctly by the _extract_nested_v3_config()
function added in commit 78be0887 (issue #4300). The test confirms the fix
works as expected and prevents future regressions.
Tags: @tdd_issue @tdd_issue_4321
ISSUES CLOSED: #4321
CoreRasurae
