test: start mocked client with no current scan to prevent mock leakage into scan_item

[wyzula-jan]

Description

There was leak of scan_item in tests, which would look like this in CI (it would pass anyway but should be fixed):

Traceback (most recent call last):
  File "/home/runner/work/bec_widgets/bec_widgets/bec_widgets/bec_widgets/widgets/plots/waveform/waveform.py", line 1673, in update_sync_curves
    x_data = self._get_x_data(device_name, device_entry)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/bec_widgets/bec_widgets/bec_widgets/bec_widgets/widgets/plots/waveform/waveform.py", line 2255, in _get_x_data
    signal_x = self.entry_validator.validate_signal(device_x, None)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/bec_widgets/bec_widgets/bec_widgets/bec_widgets/utils/entry_validator.py", line 17, in validate_signal
    raise ValueError(f"Device '{name}' not found in current BEC session")
ValueError: Device '<MagicMock name='ScanManager().scan_storage.current_scan.metadata.__getitem__().__getitem__()' id='140115129984272'>' not found in current BEC session

[Copilot]

Pull request overview

This PR updates the shared mocked_client test fixture so a freshly mocked BEC session starts with queue.scan_storage.current_scan = None, preventing MagicMock “current scan” objects from being unintentionally adopted as real scan_items by widgets and later causing noisy async Qt-slot tracebacks during unit tests.

Changes:

• Initialize bec_dispatcher.client.queue.scan_storage.current_scan to None in the shared mocked_client fixture.
• Add an explanatory comment documenting why the fixture sets current_scan to None.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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