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.

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."
+