Add a dummy viewer as a minimal implementation example

[mwcraig]

The main reason for adding this -- and I don't really care if it stays in this package long-term -- is that it helps expose some of the unstated assumptions in the API (like interactions between the states of different settings, see #12) and provides, in the test file, an example of how downstream classes would demonstrate API compatibility.

[Copilot]

Pull Request Overview

This PR introduces a dummy viewer implementation to serve as an example and exercise the API assumptions detailed in issue #12. Key changes include:

• Importing and using the dummy viewer in tests
• Adding a new test function to verify the dummy viewer instance
• Creating a test case class that inherits from the widget API test suite

[Copilot]

[nitpick] Consider adding a docstring to the test_instance function to clarify its purpose for future maintainers.

Suggested change

	def test_instance():
	def test_instance():
	"""
	Test that an instance of ImageViewer conforms to the ImageViewerInterface.
	"""

[Copilot]

[nitpick] Adding a brief docstring for TestDummyViewer could help clarify its role and intended usage, improving future maintainability.

Suggested change

	class TestDummyViewer(ImageWidgetAPITest):
	class TestDummyViewer(ImageWidgetAPITest):
	"""
	A test implementation of ImageWidgetAPITest that uses the ImageViewer
	class as the widget under test.
	"""

[Copilot]

Pull Request Overview

This PR adds a dummy viewer as a minimal implementation example to expose unstated API assumptions and provide downstream API compatibility tests.

• Adds a new test file (test_dummy_viewer.py) that verifies ImageViewer implements ImageViewerInterface and conforms to the widget API.
• Updates tests/test_astro_image_display_api.py by removing an extraneous line to clean up the file.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/test_dummy_viewer.py	Adds tests to verify implementation and API usage
tests/test_astro_image_display_api.py	Removes a stray line to tidy up the test file