test: clarify override helper cleanup

Agent-Logs-Url: https://github.com/plugboard-dev/plugboard/sessions/9de3125e-8da3-432a-a1b1-73188d656af3

Co-authored-by: toby-coleman <13170610+toby-coleman@users.noreply.github.com>

Author: Copilot

tests/conftest.py

@@ -23,7 +23,7 @@
 
 @contextmanager
 def override_settings(settings: Settings) -> _t.Iterator[None]:
-    """Temporarily override DI settings for a test."""
+    """Temporarily override DI settings for a test and always reset the override."""
     DI.settings.override_sync(settings)
     try:
         yield