TIER-2 READ conductor/code_styleguides/error_handling.md end-to-end before Phase 7: refactor(gui_2): migrate L4321 worker to Result[T] (Phase 7)

Migrate the worker() closure in _check_auto_refresh_context_preview (L4321)
to the canonical Result[T] pattern:

- Extract _worker_context_preview_result(app) -> Result[None] helper into
  new Phase 7 Worker/Background Result Helpers region.
- The legacy worker() wrapper calls the helper and drains errors to
  app.controller._worker_errors (with controller._worker_errors_lock
  acquired on append) per sub-track 3 Phase 6 Group 6.5 telemetry drain.
- The try/finally cleanup (setting _is_generating_preview=False and
  handling _pending_preview_refresh) is preserved verbatim.

Tests:
- test_phase_7_l4321_worker_context_preview_result_success
- test_phase_7_l4321_worker_context_preview_result_failure

Audit: INTERNAL_BROAD_CATCH count in src/gui_2.py dropped from 3 to 2
(remaining: L591 _diag_layout_state, L897 _capture_workspace_profile).

The lock-protected append ensures thread-safety when multiple worker
threads call _report-style drains concurrently. The helper preserves
the original fallback behavior (app.context_preview_text =
'Error generating context preview.' on failure) so the user-visible
UX is unchanged.

src/gui_2.py

@@ -4315,16 +4315,16 @@ def _check_auto_refresh_context_preview(app: App) -> None:
   app._is_generating_preview = True
   def worker():
    try:
-    app.controller.context_files = app.context_files
-    res                          = app.controller._do_generate()
-    app.context_preview_text     = res[0]
-   except Exception:
-    app.context_preview_text = "Error generating context preview."
+    result = _worker_context_preview_result(app)
+    if not result.ok:
+     if app.controller is not None and hasattr(app.controller, "_worker_errors"):
+      with app.controller._worker_errors_lock:
+       app.controller._worker_errors.append(("_check_auto_refresh_context_preview.worker", result.errors[0]))
    finally:
     app._is_generating_preview = False
     if getattr(app, "_pending_preview_refresh", False):
      app._pending_preview_refresh = False
-     # This will trigger again on next GUI frame because _last_context_preview_state 
+     # This will trigger again on next GUI frame because _last_context_preview_state
      # will be slightly behind if another change happened during the thread.
      # Or we just clear the state so it re-triggers.
      app._last_context_preview_state = None
@@ -8073,4 +8073,33 @@ def _render_beads_tab_list_result(app: "App") -> Result[list]:
 
 #endregion: Phase 5 Event Handler Result Helpers
 
+#region: Phase 7 Worker/Background Result Helpers (result_migration_gui_2_20260619)
+
+def _worker_context_preview_result(app: "App") -> Result[None]:
+ """Drain-aware variant of L4321 worker closure in _check_auto_refresh_context_preview.
+
+ Extracts the try body from the worker() closure into a Result-returning helper.
+ On exception, sets app.context_preview_text to a fallback message and returns
+ Result(data=None, errors=[ErrorInfo]). On success, sets app.context_preview_text
+ to the generated markdown. The legacy worker() wrapper drains errors to
+ app.controller._worker_errors (with the controller lock acquired on append).
+
+ [C: src/gui_2.py:_check_auto_refresh_context_preview (L4321 legacy wrapper)]
+ """
+ try:
+  app.controller.context_files = app.context_files
+  res                          = app.controller._do_generate()
+  app.context_preview_text     = res[0]
+  return Result(data=None)
+ except Exception as e:
+  app.context_preview_text = "Error generating context preview."
+  return Result(data=None, errors=[ErrorInfo(
+   kind=ErrorKind.INTERNAL,
+   message=f"Worker error generating context preview: {e}",
+   source="gui_2._worker_context_preview_result",
+   original=e,
+  )])
+
+#endregion: Phase 7 Worker/Background Result Helpers
+
 #endregion: MMA

tests/test_gui_2_result.py

@@ -1398,3 +1398,53 @@ def test_phase_6_invariant_zero_sites_in_phase_6():
   f"{phase_6_site_tests}. If a Phase 6 site was added, update the "
   f"inventory and migrate it."
  )
+
+
+# =============================================================================
+# Phase 7 Tests - Migration of 1 INTERNAL_BROAD_CATCH worker site to Result[T]
+# The helper wraps the try body from the worker() closure in
+# _check_auto_refresh_context_preview (L4321).
+# The legacy wrapper drains errors to app.controller._worker_errors (with lock).
+# =============================================================================
+
+
+def test_phase_7_l4321_worker_context_preview_result_success():
+ """
+ L4321 _worker_context_preview_result returns Result(data=None) on success.
+
+ The helper wraps the try body from the worker() closure in
+ _check_auto_refresh_context_preview. On success, sets
+ app.context_preview_text to the generated markdown and returns
+ Result(data=None).
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock
+ app = MagicMock()
+ app.controller._do_generate.return_value = ("# Generated Preview\n\nContent here", "preview.md")
+ result = gui_2._worker_context_preview_result(app)
+ assert result.ok, f"Expected ok=True on success, got errors: {result.errors}"
+ assert result.data is None
+ assert app.controller.context_files == app.context_files
+ assert app.context_preview_text == "# Generated Preview\n\nContent here"
+
+
+def test_phase_7_l4321_worker_context_preview_result_failure():
+ """
+ L4321 _worker_context_preview_result returns Result(data=None, errors=[ErrorInfo]) on failure.
+
+ When the underlying controller._do_generate() call raises, the helper sets
+ app.context_preview_text to a fallback error message and returns Result
+ with ErrorInfo describing the failure.
+ """
+ from src import gui_2
+ from unittest.mock import MagicMock
+ app = MagicMock()
+ app.controller._do_generate.side_effect = RuntimeError("do_generate blew up")
+ result = gui_2._worker_context_preview_result(app)
+ 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._worker_context_preview_result"
+ assert "do_generate blew up" in err.message
+ assert app.context_preview_text == "Error generating context preview."
+