TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 5: refactor(gui_2): migrate L1428 request_patch_from_tier4 to Result[T] (Phase 5)

Extract request_patch_from_tier4_result helper from the
ai_client.run_tier4_patch_generation try/except in App.request_patch_from_tier4.
Legacy wrapper drains errors to app._last_request_errors per FR-BC-4
event-handler pattern.

[pre-audit] L1428 INTERNAL_BROAD_CATCH
[post-audit] V count: 10 -> 9 (L1428 removed)

tests/test_gui_2_result.py

@@ -975,3 +975,46 @@ def test_phase_5_l1393_open_patch_in_external_editor_result_failure():
  err = result.errors[0]
  assert err.source == "gui_2._open_patch_in_external_editor_result"
  assert "temp file creation blew up" in err.message
+
+
+def test_phase_5_l1428_request_patch_from_tier4_result_success():
+ """
+ L1428 request_patch_from_tier4_result returns Result.ok=True on success.
+
+ The helper wraps the ai_client.run_tier4_patch_generation try/except in
+ App.request_patch_from_tier4. On success (patch_text has --- and +++),
+ returns Result(data=True) and sets app._pending_patch_text/files and
+ app._show_patch_modal=True.
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock, patch
+ app = MagicMock()
+ patch_text = "--- a/foo.py\n+++ b/foo.py\n@@ -1 +1 @@\n-old\n+new\n"
+ mock_diff_files = [MagicMock(old_path="/proj/foo/foo.py", new_path="/proj/foo/foo.py")]
+ with patch.object(gui_2.ai_client, "run_tier4_patch_generation", return_value=patch_text), \
+   patch("src.diff_viewer.parse_diff", return_value=mock_diff_files):
+  result = gui_2.request_patch_from_tier4_result(app, "boom", "/proj/foo/foo.py")
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is True
+ assert app._pending_patch_text is patch_text
+ assert app._pending_patch_files == ["/proj/foo/foo.py"]
+ assert app._show_patch_modal is True
+
+
+def test_phase_5_l1428_request_patch_from_tier4_result_failure():
+ """
+ L1428 request_patch_from_tier4_result returns Result.ok=False with ErrorInfo on failure.
+
+ When ai_client.run_tier4_patch_generation 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()
+ with patch.object(gui_2.ai_client, "run_tier4_patch_generation", side_effect=RuntimeError("tier4 backend down")):
+  result = gui_2.request_patch_from_tier4_result(app, "boom", "/proj/foo/foo.py")
+ 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.request_patch_from_tier4_result"
+ assert "tier4 backend down" in err.message