Demo mode crashes with `ModuleNotFoundError: No module named 'pytest'` in MSI-installed builds

[ThomasMalletCodra]

Describe the bug

In a DataLab instance installed via the MSI installer (frozen/PyInstaller build), triggering Demo mode (toolbar action or menu entry) crashes with a traceback instead of running the demo scenario:

Traceback (most recent call last):
  File "guidata\qthelpers.py", line 377, in <lambda>
  File "datalab\gui\main.py", line 2308, in play_demo
  File "pyimod02_importers.py", line 457, in exec_module
  File "datalab\tests\__init__.py", line 33, in <module>
ModuleNotFoundError: No module named 'pytest'

Root cause (identified in code)

DLMainWindow.play_demo() in datalab/gui/main.py (line 2304) calls
datalab.tests.scenarios.demo.play_demo(), which does
from datalab.tests import datalab_test_app_context (datalab/tests/scenarios/demo.py,
line 31). This triggers the import of datalab/tests/__init__.py, which has
import pytest at module level (line 33).

Since pytest is a test-only dependency, it is not bundled by PyInstaller in the
MSI/standalone build, so any code path that imports datalab.tests (even indirectly,
just to reuse the demo/test-context helper) fails at runtime in frozen installations.

To Reproduce

1. Install DataLab using the official MSI installer (not from a development/pip
   environment with pytest available).
2. Launch DataLab.
3. Trigger Demo mode (toolbar action or menu entry calling play_demo).
4. Observe the ModuleNotFoundError: No module named 'pytest' traceback (visible in
   the console, or via the error dialog/log).

Expected behavior

The Demo mode should run correctly in installed/frozen builds, without requiring
pytest (a development/test dependency) to be present.

Environment

• Installation type: MSI installer (frozen/PyInstaller build)
• Affected versions: 1.1.0, 1.2.2, 1.3.0 (confirmed on multiple releases — long-standing
  regression)
• OS: Windows

Possible fix directions

• Decouple datalab.tests.scenarios.demo from datalab.tests.__init__ (e.g., move
  datalab_test_app_context to a module that doesn't import pytest), or
• Make the pytest import in datalab/tests/__init__.py lazy/optional, or
• Bundle pytest as a hidden import in the PyInstaller spec (less desirable, increases
  installer size for a test-only dependency).