TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 5: refactor(gui_2): migrate L5786 render_text_viewer_window ced to Result[T] (Phase 5)

Extract _render_text_viewer_window_ced_result helper from the
TextEditor set_text/render try/except in render_text_viewer_window CED
branch. Legacy wrapper drains errors to app._last_request_errors per FR-BC-4
event-handler pattern.

[pre-audit] L5786 INTERNAL_BROAD_CATCH
[post-audit] V count: 6 -> 5 (L5786 removed)

tests/test_gui_2_result.py

@@ -1142,3 +1142,46 @@ def test_phase_5_l5380_render_operations_hub_external_editor_panel_result_failur
  err = result.errors[0]
  assert err.source == "gui_2._render_operations_hub_external_editor_panel_result"
  assert "ext editor render blew up" in err.message
+
+
+def test_phase_5_l5786_render_text_viewer_window_ced_result_success():
+ """
+ L5786 _render_text_viewer_window_ced_result returns Result.ok=True on success.
+
+ The helper wraps the TextEditor set_text/render try/except in
+ render_text_viewer_window. On success, returns Result(data=True).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app = MagicMock()
+ app.perf_profiling_enabled = False
+ app.text_viewer_content   = "line1\nline2\n"
+ app.text_viewer_title     = "test.txt"
+ app._text_viewer_editor   = MagicMock()
+ result = gui_2._render_text_viewer_window_ced_result(app)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is True
+
+
+def test_phase_5_l5786_render_text_viewer_window_ced_result_failure():
+ """
+ L5786 _render_text_viewer_window_ced_result returns Result.ok=False with ErrorInfo on failure.
+
+ When TextEditor render raises, the helper converts the exception to ErrorInfo
+ and returns Result(data=False, errors=[ErrorInfo]).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app = MagicMock()
+ app.perf_profiling_enabled = False
+ app.text_viewer_content   = "line1\nline2\n"
+ app.text_viewer_title     = "test.txt"
+ mock_editor = MagicMock()
+ mock_editor.set_text.side_effect = RuntimeError("ced set_text failed")
+ app._text_viewer_editor   = mock_editor
+ result = gui_2._render_text_viewer_window_ced_result(app)
+ assert not result.ok, f"Expected ok=False on failure, got data: {result.data}"
+ assert result.errors, "Expected at least one error on failure"
+ err = result.errors[0]
+ assert err.source == "gui_2._render_text_viewer_window_ced_result"
+ assert "ced set_text failed" in err.message