fix(rag): reset rag_config to default RAGConfig() (not None) in _handle_reset_session

src/app_controller.py

@@ -3393,8 +3393,14 @@ class AppController:
   # Reset RAG engine state so the chroma collection from a prior test
   # doesn't leak into the next session. The next _sync_rag_engine will
   # rebuild the engine with the new active_project_root.
+  # NOTE: rag_config is reset to a fresh default (not None) so the rag_*
+  # setters can still mutate it. Setting it to None was a bug — setters
+  # check `if self.rag_config:` and become no-ops, leaving the engine
+  # unsynced forever (test_rag_phase4_final_verify regression on
+  # 2026-06-10).
   self.rag_engine = None
-  self.rag_config = None
+  from src import models as _rag_models
+  self.rag_config = _rag_models.RAGConfig()
   self.rag_status = 'idle'
   self._rag_sync_token = 0
   self._rag_sync_dirty = False