Use headless viewers in layer and save widget unit tests

[SahilKumar75]

Description

Move the existing headless napari viewer fixture into the shared fixture module and use it for the layer style tests that only exercise Shapes layer state.

This reduces unnecessary Qt viewer construction while preserving the Qt backed fixture for widget lifecycle and integration coverage.

Motivation and Context

This is the first focused part of issue 1111. The migrated tests only require ViewerModel and do not interact with Qt widgets.

Closes no issue. Progresses #1111.

How Has This Been Tested?

pytest tests/test_unit/test_napari_plugin/test_layer_styles.py
29 passed

pytest tests/test_unit/test_napari_plugin/test_layer_wiring.py::test_frame_slider_range_covers_nan_trimmed_frames tests/test_unit/test_napari_plugin/test_layer_wiring.py::test_frame_slider_range_w_non_movement_layers tests/test_unit/test_napari_plugin/test_layer_wiring.py::test_frame_slider_range_ignores_row_count
5 passed

pre-commit run --files tests/fixtures/napari.py tests/test_unit/test_napari_plugin/test_layer_styles.py tests/test_unit/test_napari_plugin/test_layer_wiring.py
All hooks passed

The complete layer wiring module reaches unchanged Qt viewer tests and then crashes during macOS OpenGL initialization. The migrated headless tests pass independently.

Additional context

The repository does not contain a pull request template. This change follows the contribution guide by keeping the scope focused, linking the issue, and reporting exact validation results.

[SahilKumar75]

Extended this to eight save widget unit cases and the callback connection test. The save widget still uses real Qt buttons and signals, with qtbot handling cleanup. Only the viewer is headless.

File round trips, save failures and docked widget lifecycle tests still use the full viewer.

All 352 plugin tests passed locally on macOS with napari 0.9.2. Repository hooks passed too. The earlier local OpenGL blocker did not recur in this native Qt run.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud