test(gui_2): add 2 Phase 5 invariant tests + Phase 5 checkpoint

TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 5.

Phase 5 Batch C migration complete. 11 INTERNAL_BROAD_CATCH event-handler
sites migrated to Result[T] pattern per FR-BC-4. The legacy wrappers drain
errors to app._last_request_errors (data plane attribute).

Migrated sites:
- L1284 _populate_auto_slices outline
- L1293 _populate_auto_slices file_read
- L1367 _apply_pending_patch
- L1393 _open_patch_in_external_editor
- L1428 request_patch_from_tier4
- L3163 render_tool_preset_manager_content bias_save
- L3582 render_context_batch_actions preview
- L5380 render_operations_hub ext_editor_panel
- L5786 render_text_viewer_window ced
- L5920 render_external_editor_panel config
- L7208 render_beads_tab list

V count dropped from 14 to 3 (11 sites migrated; remaining 3 in Phase 7/8).

Invariant tests:
- test_phase_5_invariant_batch_c_count_dropped: locks V count <= 3
- test_phase_5_invariant_all_11_migration_sites_have_tests: locks all 11
  sites have both success and failure tests

tests/test_gui_2_result.py

@@ -1270,3 +1270,63 @@ def test_phase_5_l7208_render_beads_tab_list_result_failure():
  err = result.errors[0]
  assert err.source == "gui_2._render_beads_tab_list_result"
  assert "dolt backend down" in err.message
+
+
+# =============================================================================
+# Phase 5 Invariant Tests (result_migration_gui_2_20260619)
+# Lock the per-phase progress: 11 INTERNAL_BROAD_CATCH event-handler sites
+# migrated, all have both success and failure tests.
+# =============================================================================
+
+
+def test_phase_5_invariant_batch_c_count_dropped():
+ """
+ Phase 5 invariant: the audit's INTERNAL_BROAD_CATCH count for src/gui_2.py
+ has dropped from 14 (pre-Phase 5) to 3 (post-Phase 5). The 3 remaining sites
+ are in other phases: L591 (Phase 8), L897 (Phase 8), L4321 (Phase 7).
+
+ The 11 migrated sites are: L1284, L1293, L1367, L1393, L1428, L3163, L3582,
+ L5380, L5786, L5920, L7208.
+ """
+ result = subprocess.run(
+  ["uv", "run", "python", "scripts/audit_exception_handling.py", "--src", "src", "--json"],
+  capture_output=True,
+  text=True,
+ )
+ assert result.returncode == 0, (
+  f"audit_exception_handling.py exited {result.returncode}; stderr:\n"
+  f"{result.stderr[:2000]}"
+ )
+ data = json.loads(result.stdout)
+ gui2 = [f for f in data.get("files", []) if "gui_2" in f.get("filename", "")][0]
+ broad_catches = [f for f in gui2.get("findings", []) if f.get("category") == "INTERNAL_BROAD_CATCH"]
+ # Pre-Phase 5 baseline: 14. Post-Phase 5: 3 (11 sites migrated).
+ # Per FR-BC-4, the 11 migrated sites drain to app._last_request_errors.
+ assert len(broad_catches) <= 3, (
+  f"Phase 5 invariant: expected <= 3 INTERNAL_BROAD_CATCH sites in src/gui_2.py "
+  f"(post-Phase 5 baseline, 11 sites migrated); found {len(broad_catches)}. "
+  f"The 11 Phase 5 Batch C sites (L1284, L1293, L1367, L1393, L1428, L3163, "
+  f"L3582, L5380, L5786, L5920, L7208) must be migrated to Result[T] helpers. "
+  f"Lines: {[f.get('line') for f in broad_catches]}"
+ )
+
+
+def test_phase_5_invariant_all_11_migration_sites_have_tests():
+ """
+ Phase 5 invariant: each of the 11 Batch C sites has both success and
+ failure tests in this test file.
+ """
+ import re
+ text = Path(__file__).read_text(encoding="utf-8")
+ expected_lines = [1284, 1293, 1367, 1393, 1428, 3163, 3582, 5380, 5786, 5920, 7208]
+ for line in expected_lines:
+  success_pattern = f"test_phase_5_l{line}_.*_result_success"
+  failure_pattern = f"test_phase_5_l{line}_.*_result_failure"
+  assert re.search(success_pattern, text), (
+   f"Phase 5 invariant: missing success test for L{line}. "
+   f"Expected a test matching '{success_pattern}'."
+  )
+  assert re.search(failure_pattern, text), (
+   f"Phase 5 invariant: missing failure test for L{line}. "
+   f"Expected a test matching '{failure_pattern}'."
+  )