fix(dashboard): repair stale get_active_modes test double

_DummyAgent.get_active_modes() returned a bare list, but the real
SerenaAgent.get_active_modes() returns an ActiveModes instance and
dashboard._compute_config_overview() calls .get_modes() on it, so
test_config_overview_includes_tool_stats_totals raised AttributeError.
Return a stub exposing get_modes() to mirror the real API.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: pavlo-basanets-epam

test/serena/test_dashboard.py

@@ -58,7 +58,9 @@ def get_context(self):
         return _DummyContext()
 
     def get_active_modes(self):
-        return []
+        # mirror the real SerenaAgent.get_active_modes(), which returns an ActiveModes
+        # instance exposing get_modes(); dashboard._compute_config_overview() calls it
+        return SimpleNamespace(get_modes=lambda include_background_base_modes=False: [])
 
     def get_active_tool_names(self):
         return []