fix(tui_v3): silence Renderer's cpr_not_supported_callback too

The Application's cpr_not_supported_callback override was not
enough — prompt-toolkit's Renderer captures its own copy during
__init__ and calls that one when CPR times out.  The warning wrote
directly to the real terminal fd (via self.output.write()), causing
a brief flash before the TUI overwrote it on the next frame.

Silence both callbacks (app + renderer) to fully suppress the flash.

Co-Authored-By: GenericAgent <bot@gaagent.ai>

Author: drac

frontends/tui_v3.py

@@ -5982,7 +5982,12 @@ def _pre_run() -> None:
             before_render=_before_render,
             output=_ptk_output,
         )
-        app.cpr_not_supported_callback = lambda: None  # silence the CPR warning
+        # Silence the CPR warning.  The Application's callback is checked by
+        # prompt_toolkit's Application.run() path, but the Renderer captures its
+        # own copy during __init__ and that is the one that actually fires the
+        # warning.  Both must be no-ops to prevent the brief flash on startup.
+        app.cpr_not_supported_callback = lambda: None
+        app.renderer.cpr_not_supported_callback = lambda: None
         self._ptk_app = app
 
         try: