fix(phase3): Resolve remaining test failures and stabilize GUI

- Fixed
ullcontext NameError in gui_2.py.
- Corrected TestMMAApprovalIndicators to call real rendering methods on mock app.
- Updated 	est_history_manager.py to provide required context_files argument to UISnapshot.
- Stabilized 	est_z_negative_flows.py with robust polling for terminal response status and corrected field names.
- Cleaned up debug logging in 
ag_engine.py and pp_controller.py.

src/gui_2.py

@@ -17,7 +17,7 @@ import threading
 import time
 import tomli_w
 import typing
-# from contextlib import ExitStack, nullcontext
+from contextlib import ExitStack, nullcontext
 from defer import defer
 from imgui_bundle import imgui, hello_imgui, immapp, imgui_node_editor as ed, imgui_color_text_edit as ced
 from pathlib import Path
@@ -771,7 +771,7 @@ class App:
   try:
    self._render_main_interface()
   except Exception as e:
-   print(f"ERROR in _gui_func: {e}")
+   sys.stderr.write(f"ERROR in _gui_func: {e}\n")
    traceback.print_exc()
   
   if pushed_prior_tint: