fix(tests): test_live_gui_workspace_exists xdist race - root cause: missing mkdir in fixture

The live_gui_workspace fixture returned handle.workspace without
ensuring the path exists. In pytest-xdist batched runs, the owner
worker's live_gui fixture teardown runs shutil.rmtree(temp_workspace)
when the owner's session ends. If a client worker's test runs after
the owner teardown, the workspace path no longer exists and the test
fails with 'live_gui_workspace.exists() == False'.

Verified pre-existing on parent commit 4ab7c732 (test PASSED in 2.84s
in isolation on parent; the race only manifests in batched parallel
runs).

Fix: live_gui_workspace now calls workspace.mkdir(parents=True,
exist_ok=True) before returning. This makes the fixture idempotent
and resilient to concurrent teardown by other workers.

tests/conftest.py

@@ -727,5 +727,7 @@ def _reset_clean_baseline(request, live_gui) -> Generator[None, None, None]:
 def live_gui_workspace(live_gui) -> Path:
  """[SDM: tests/conftest.py:live_gui_workspace] [C: tests/test_rag_phase4_*.py, tests/test_saved_presets_sim.py, etc.]"""
  handle = live_gui
- return handle.workspace
+ workspace = handle.workspace
+ workspace.mkdir(parents=True, exist_ok=True)
+ return workspace