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