TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 10: refactor(gui_2): migrate L7271 render_task_dag_panel cycle_check to Result[T] (Phase 10 site 12)

Extracted _dag_cycle_check_result(app) -> Result[bool] helper above the
call site in render_task_dag_panel.
ANTI-SLIMING: full Result[T] propagation (NO except+pass). The helper
returns Result(data=has_cycle) on success (True/False) or
Result(data=False, errors=[ErrorInfo]) on exception (logging NOT a drain
per the user's principle 2026-06-17).

The legacy render_task_dag_panel code preserves its signature, calls the
helper, opens the 'Cycle Detected!' popup only when the helper returns
Result(data=True), and drains errors to app._last_request_errors.

Tests: 3 new tests verify no-cycle, cycle-detected, and RuntimeError paths.

Audit: L7271 reclassified from INTERNAL_SILENT_SWALLOW (1 site remaining,
was 2). New helper L7271 is INTERNAL_COMPLIANT.

tests/test_gui_2_result.py

@@ -2202,4 +2202,69 @@ def test_phase_10_l6908_tier_stream_scroll_sync_result_failure():
  assert "missing key" in err.message
 
 
+def test_phase_10_l7271_dag_cycle_check_result_no_cycle():
+ """
+ L7271 _dag_cycle_check_result returns Result(data=False) when no cycle is found.
+
+ The helper extracts the TrackDAG() cycle check try/except from
+ render_task_dag_panel into a Result-returning helper. On a valid DAG
+ (no cycle), returns Result(data=False). The caller continues without
+ opening the "Cycle Detected!" popup.
+ """
+ from unittest.mock import MagicMock, patch
+ import src.gui_2 as gui2_mod
+ app = MagicMock()
+ app.active_tickets = [{"id": "T-001", "depends_on": []}]
+ mock_dag = MagicMock()
+ mock_dag.has_cycle.return_value = False
+ with patch("src.dag_engine.TrackDAG", return_value=mock_dag):
+  result = gui2_mod._dag_cycle_check_result(app)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is False
+
+
+def test_phase_10_l7271_dag_cycle_check_result_cycle_detected():
+ """
+ L7271 _dag_cycle_check_result returns Result(data=True) when a cycle IS found.
+
+ The helper detects cycles via TrackDAG.has_cycle(). On a cyclic DAG,
+ returns Result(data=True). The caller opens the "Cycle Detected!" popup.
+ """
+ from unittest.mock import MagicMock, patch
+ import src.gui_2 as gui2_mod
+ app = MagicMock()
+ app.active_tickets = [
+  {"id": "T-001", "depends_on": ["T-002"]},
+  {"id": "T-002", "depends_on": ["T-001"]},
+ ]
+ mock_dag = MagicMock()
+ mock_dag.has_cycle.return_value = True
+ with patch("src.dag_engine.TrackDAG", return_value=mock_dag):
+  result = gui2_mod._dag_cycle_check_result(app)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is True
+
+
+def test_phase_10_l7271_dag_cycle_check_result_failure():
+ """
+ L7271 _dag_cycle_check_result returns Result(data=False, errors=[ErrorInfo]) on failure.
+
+ When TrackDAG() construction or has_cycle() raises (bad ticket dict,
+ dag engine error), the helper converts to ErrorInfo. The caller
+ drains to app._last_request_errors.
+ """
+ from unittest.mock import MagicMock, patch
+ import src.gui_2 as gui2_mod
+ app = MagicMock()
+ app.active_tickets = []
+ with patch("src.dag_engine.TrackDAG", side_effect=RuntimeError("dag engine failure")):
+  result = gui2_mod._dag_cycle_check_result(app)
+ assert not result.ok, f"Expected ok=False on failure, got data: {result.data}"
+ assert result.data is False
+ assert result.errors, "Expected at least one error on failure"
+ err = result.errors[0]
+ assert err.source == "gui_2._dag_cycle_check_result"
+ assert "dag engine failure" in err.message
+
+