Prevent PresetsWidget internal refreshes from applying configs

[gcharvin]

Summary

• Block PresetsWidget combo signals during the full internal preset-state refresh.
• Prevent adding/removing <no match> from triggering setConfig().
• Add a regression test for the <no match> -> preset transition.

Motivation

When PresetsWidget refreshes after core property changes, removing the currently selected <no match> item can emit currentTextChanged. Since _on_combo_changed() calls mmc.setConfig(), an internal UI refresh may accidentally apply a hardware preset.

This was observed during MDA channel switching, where transient non-matching device states could cause an unintended channel preset to be selected. The issue is generic Qt signal reentrancy, not microscope-specific.

There is no screenshot attached because this is not a visual rendering change: the problematic behavior is an internal Qt signal emitted during combo-box synchronization. The regression test captures the behavior directly by asserting that the UI refresh does not emit configSet.

Fix

Wrap the entire _update_if_props_not_match_preset() combo synchronization in signals_blocked(self._combo), including removeItem, addItem, and setCurrentText.

This matches the existing pattern used by ChannelGroupWidget.

Tests

• ruff check src/pymmcore_widgets/control/_presets_widget.py tests/test_presets_widget.py
• ruff format --check src/pymmcore_widgets/control/_presets_widget.py tests/test_presets_widget.py
• pytest tests/test_presets_widget.py tests/test_channel_widget.py tests/test_group_preset_widget.py -q

[tlambert03]

looks good to me. thank you!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.43%. Comparing base (48ff414) to head (636fd02).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #558      +/-   ##
==========================================
+ Coverage   87.42%   87.43%   +0.01%     
==========================================
  Files         115      115              
  Lines       14095    14094       -1     
==========================================
+ Hits        12322    12323       +1     
+ Misses       1773     1771       -2     

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