TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 10: refactor(gui_2): migrate L1466 _close_vscode_diff terminate to Result[T] (Phase 10 site 7)

Extracted _close_vscode_diff_terminate_result(app) -> Result[None]
helper above the App._close_vscode_diff method.
ANTI-SLIMING: full Result[T] propagation (NO except+pass). The helper
returns Result(data=None) on success or Result(data=None,
errors=[ErrorInfo]) on exception (logging NOT a drain per the user's
principle 2026-06-17).

The legacy _close_vscode_diff method preserves its signature, calls
the helper, drains errors to self._last_request_errors, and proceeds
to set self._vscode_diff_process = None (preserving the original
post-error behavior of clearing the handle).

Tests: 2 new tests verify both paths (success and OSError).

Audit: L1466 reclassified from INTERNAL_SILENT_SWALLOW (7 sites remaining,
was 8). New helper L1466 is INTERNAL_COMPLIANT.

tests/test_gui_2_result.py

@@ -1990,4 +1990,46 @@ def test_phase_10_l1152_gui_func_entry_log_result_failure():
  assert "broken pipe" in err.message
 
 
+def test_phase_10_l1466_close_vscode_diff_terminate_result_success():
+ """
+ L1466 _close_vscode_diff_terminate_result returns Result(data=None) on success.
+
+ The helper extracts the self._vscode_diff_process.terminate() try/except
+ from App._close_vscode_diff into a Result-returning helper. On success,
+ returns Result(data=None). The legacy wrapper sets
+ self._vscode_diff_process = None (the original behavior preserved).
+ """
+ from unittest.mock import MagicMock
+ import src.gui_2 as gui2_mod
+ app = MagicMock()
+ app._vscode_diff_process = MagicMock()
+ result = gui2_mod._close_vscode_diff_terminate_result(app)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is None
+ app._vscode_diff_process.terminate.assert_called_once()
+
+
+def test_phase_10_l1466_close_vscode_diff_terminate_result_failure():
+ """
+ L1466 _close_vscode_diff_terminate_result returns Result(data=None, errors=[ErrorInfo]) on failure.
+
+ When the process.terminate() call raises (e.g., process already exited,
+ invalid handle), the helper converts to ErrorInfo. The legacy wrapper
+ drains to self._last_request_errors and proceeds to set
+ self._vscode_diff_process = None (preserving the original behavior).
+ """
+ from unittest.mock import MagicMock
+ import src.gui_2 as gui2_mod
+ app = MagicMock()
+ app._vscode_diff_process = MagicMock()
+ app._vscode_diff_process.terminate.side_effect = OSError("process already exited")
+ result = gui2_mod._close_vscode_diff_terminate_result(app)
+ assert not result.ok, f"Expected ok=False on failure, got data: {result.data}"
+ assert result.data is None
+ assert result.errors, "Expected at least one error on failure"
+ err = result.errors[0]
+ assert err.source == "gui_2._close_vscode_diff_terminate_result"
+ assert "process already exited" in err.message
+
+