TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 5: refactor(gui_2): migrate L1293 _populate_auto_slices file_read to Result[T] (Phase 5)

Extract _populate_auto_slices_file_read_result helper from the file read
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] L1293 INTERNAL_BROAD_CATCH
[post-audit] V count: 13 -> 12 (L1293 removed)

tests/test_gui_2_result.py

@@ -839,3 +839,42 @@ def test_phase_5_l1284_populate_auto_slices_outline_result_failure():
  err = result.errors[0]
  assert err.source == "gui_2._populate_auto_slices_outline_result"
  assert "outline fetch failed" in err.message
+
+
+def test_phase_5_l1293_populate_auto_slices_file_read_result_success():
+ """
+ L1293 _populate_auto_slices_file_read_result returns Result.ok=True on success.
+
+ The helper wraps the file read try/except in App._populate_auto_slices. On
+ success, returns Result(data=content) where content is the file text.
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch, mock_open
+ app       = MagicMock()
+ f_item    = MagicMock()
+ f_item.path = "/proj/foo/src/bar.py"
+ with patch("builtins.open", mock_open(read_data="line1\nline2\nline3")):
+  result = gui_2._populate_auto_slices_file_read_result(app, f_item)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data == "line1\nline2\nline3"
+
+
+def test_phase_5_l1293_populate_auto_slices_file_read_result_failure():
+ """
+ L1293 _populate_auto_slices_file_read_result returns Result.ok=False with ErrorInfo on failure.
+
+ When the file read raises (e.g., file not found, encoding error), 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/missing.py"
+ with patch("builtins.open", side_effect=FileNotFoundError("no such file")):
+  result = gui_2._populate_auto_slices_file_read_result(app, f_item)
+ 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_file_read_result"
+ assert "no such file" in err.message