fix(gui_2): restore _last_imgui_assert as string (regression from Phase 10)

The Phase 10 migration of the run() function (L728 INTERNAL_SILENT_SWALLOW)
changed App.run's error drain to set self.controller._last_imgui_assert
to traceback.format_exception(...), which returns a list. But the
existing test test_app_run_imgui_assert_handling.py expects it to be
a string containing 'Missing End'.

Fix: set _last_imgui_assert to str(err.original) if available, else
err.message. The IM_ASSERT message string is what the health endpoint
expects.

TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 13.

Regression test: tests/test_app_run_imgui_assert_handling.py
test_app_run_records_degraded_state_on_imgui_assert PASSES after fix.

src/gui_2.py

@@ -727,7 +727,7 @@ class App:
     err = run_result.errors[0]
     if hasattr(self, "controller") and self.controller is not None:
      self.controller._gui_degraded_reason = err.message
-     self.controller._last_imgui_assert = traceback.format_exception(type(err.original), err.original, err.original.__traceback__) if err.original else ""
+     self.controller._last_imgui_assert = str(err.original) if err.original else err.message
     if not hasattr(self, '_startup_timeline_errors'): self._startup_timeline_errors = []
     self._startup_timeline_errors.append(("run.immapp", err))
     return