TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 5: refactor(gui_2): migrate L1284 _populate_auto_slices outline to Result[T] (Phase 5)

Extract _populate_auto_slices_outline_result helper from the
mcp_client.{py,ts_c,ts_cpp}_get_code_outline try/except in
App._populate_auto_slices. Legacy wrapper drains errors to
app._last_request_errors per FR-BC-4 event-handler pattern.

[pre-audit] L1284 INTERNAL_BROAD_CATCH
[post-audit] V count: 14 -> 13 (L1284 removed)

tests/test_gui_2_result.py

@@ -790,3 +790,52 @@ def test_phase_4_invariant_all_3_migration_sites_have_tests():
    f"Phase 4 invariant: missing failure test for L{line}. "
    f"Expected a test matching '{failure_pattern}'."
   )
+
+
+# =============================================================================
+# Phase 5 Tests - Migration of 11 INTERNAL_BROAD_CATCH event-handler sites to Result[T]
+# Each site gets 2 tests: success and failure.
+# Migration pattern: legacy wrapper routes errors to app._last_request_errors (per FR-BC-4).
+# =============================================================================
+
+
+def test_phase_5_l1284_populate_auto_slices_outline_result_success():
+ """
+ L1284 _populate_auto_slices_outline_result returns Result.ok=True on success.
+
+ The helper wraps the mcp_client.{py,ts_c,ts_cpp}_get_code_outline try/except
+ in App._populate_auto_slices. On success (Python file extension matches),
+ returns Result(data=outline).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app       = MagicMock()
+ f_item    = MagicMock()
+ f_item.path = "/proj/foo/src/bar.py"
+ abs_path = "/proj/foo/src/bar.py"
+ with patch.object(gui_2.mcp_client, "py_get_code_outline", return_value="[def] foo (Lines 1-10)"):
+  result = gui_2._populate_auto_slices_outline_result(app, f_item, abs_path)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data == "[def] foo (Lines 1-10)"
+
+
+def test_phase_5_l1284_populate_auto_slices_outline_result_failure():
+ """
+ L1284 _populate_auto_slices_outline_result returns Result.ok=False with ErrorInfo on failure.
+
+ When the underlying mcp_client outline fetch raises, the helper converts the
+ exception to ErrorInfo and returns Result(data="", errors=[ErrorInfo]).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app       = MagicMock()
+ f_item    = MagicMock()
+ f_item.path = "/proj/foo/src/bar.py"
+ abs_path = "/proj/foo/src/bar.py"
+ with patch.object(gui_2.mcp_client, "py_get_code_outline", side_effect=RuntimeError("outline fetch failed")):
+  result = gui_2._populate_auto_slices_outline_result(app, f_item, abs_path)
+ 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._populate_auto_slices_outline_result"
+ assert "outline fetch failed" in err.message