TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 4: test(gui_2): add 2 Phase 4 invariant tests + relax Phase 3 invariant for decreasing count

The Phase 3 invariant test (test_phase_3_invariant_batch_a_count_dropped)
asserted exactly 17 INTERNAL_BROAD_CATCH sites, the post-Phase 3 baseline.
After Phase 4 migrates 3 more sites, the count drops to 14. The test now
asserts <= 17 (the upper bound; the Phase 3 boundary).

Adds test_phase_4_invariant_batch_b_count_dropped: locks in <= 14 sites
(post-Phase 4 baseline; down from 17).

Adds test_phase_4_invariant_all_3_migration_sites_have_tests: ensures each
of the 3 Batch B sites (L3398, L3718, L3740) has both _success and _failure tests.

All 30 tests pass.

tests/test_gui_2_result.py

@@ -505,10 +505,13 @@ def test_phase_3_invariant_batch_a_count_dropped():
  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"]
- assert len(broad_catches) == 17, (
-  f"Phase 3 invariant: expected 17 INTERNAL_BROAD_CATCH sites in src/gui_2.py "
-  f"(down from 25); found {len(broad_catches)}. The 8 Batch A sites must be "
-  f"migrated to Result[T] helpers. Lines: {[f.get('line') for f in broad_catches]}"
+ # Post-Phase 3 baseline was 17. As subsequent phases (Phase 4+) migrate more
+ # sites, the count decreases. This test asserts the upper bound (the Phase 3
+ # boundary); per-phase invariant tests track the decreasing count.
+ assert len(broad_catches) <= 17, (
+  f"Phase 3 invariant: expected <= 17 INTERNAL_BROAD_CATCH sites in src/gui_2.py "
+  f"(post-Phase 3 baseline); found {len(broad_catches)}. The count grew, which "
+  f"means a regression or new site was introduced. Lines: {[f.get('line') for f in broad_catches]}"
  )
 
 
@@ -737,3 +740,53 @@ def test_phase_4_l3740_render_ast_inspector_file_content_result_failure():
  err = result.errors[0]
  assert err.source == "gui_2._render_ast_inspector_file_content_result"
  assert result.data is None
+
+
+def test_phase_4_invariant_batch_b_count_dropped():
+ """
+ Phase 4 invariant: the audit's INTERNAL_BROAD_CATCH count for src/gui_2.py
+ has dropped from 17 (post-Phase 3) to 14 (a drop of 3 sites).
+
+ The 3 migrated sites are: L3398 (render_persona_editor_window Save),
+ L3718 (render_ast_inspector_modal outline fetch),
+ L3740 (render_ast_inspector_modal file content read).
+ """
+ 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"]
+ assert len(broad_catches) <= 14, (
+  f"Phase 4 invariant: expected <= 14 INTERNAL_BROAD_CATCH sites in src/gui_2.py "
+  f"(post-Phase 4 baseline, down from 17); found {len(broad_catches)}. "
+  f"The 3 Batch B sites (L3398, L3718, L3740) must be migrated to Result[T] helpers. "
+  f"Lines: {[f.get('line') for f in broad_catches]}"
+ )
+
+
+def test_phase_4_invariant_all_3_migration_sites_have_tests():
+ """
+ Phase 4 invariant: each of the 3 Batch B sites has both success and
+ failure tests in this test file.
+ """
+ import re
+ text = Path(__file__).read_text(encoding="utf-8")
+ expected_lines = [3398, 3718, 3740]
+ for line in expected_lines:
+  success_pattern = f"test_phase_4_l{line}_.*_success"
+  failure_pattern = f"test_phase_4_l{line}_.*_failure"
+  assert re.search(success_pattern, text), (
+   f"Phase 4 invariant: missing success test for L{line}. "
+   f"Expected a test matching '{success_pattern}'."
+  )
+  assert re.search(failure_pattern, text), (
+   f"Phase 4 invariant: missing failure test for L{line}. "
+   f"Expected a test matching '{failure_pattern}'."
+  )