Clean up test suite

[TimMonko]

Description

Generally cleans up the test suite and speeds it up by having fewer make_napari_viewer() tests.
The tests were mostly internal duplications, in the wrong file, or testing the same thing as another test.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

This PR restructures the test suite to improve organization and performance by reducing unnecessary napari viewer instantiations. Tests are reorganized into logical modules that better reflect the code structure they test, with duplicate tests removed and similar tests consolidated using parametrization.

Key changes:

• Created dedicated test modules (test_plugin_manager.py, test_plugin_installer_widget.py) for better separation of concerns
• Reduced viewer fixture usage in tests that don't require napari integration, improving test execution speed
• Consolidated duplicate tests using pytest parametrization for cleaner, more maintainable test code

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/test_plugin_manager.py	New file testing ReaderPluginManager functionality with proper mocking, no viewer dependency
tests/test_plugin_installer_widget.py	New file separating widget unit tests (no viewer) from integration tests (with viewer)
tests/test_plugin_installer_integration.py	Deleted - tests moved to appropriate new files with better organization
tests/test_plugin_installer.py	Refactored to focus on _plugin_installer module, removed tests that belong elsewhere, cleaned up comments
tests/test_nimage.py	Removed duplicate assertions (lines 206-208) and test_determine_reader_plugin_behavior (covered elsewhere)
tests/test_napari_reader.py	Removed unnecessary make_napari_viewer parameter from test_reader_supported_formats, deleted duplicate test_napari_get_reader_png
tests/test_bioio_plugin_utils.py	Refactored individual tests into parametrized tests for better coverage and reduced duplication
pixi.lock	Updated dependency lock with version bump and addition of pooch dependency

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