fix: ensure default store data is merged with persisted data during VisualizationStore initialization

[madalynrose]

Details

When there is persisted data in IndexedDB for the VisualizationStore and storeData.tests is missing the mediumPass key, the WebConfigurationVisualizationFactory hits an error. This PR overrides generateDefaultState for VisualizationStore (called during initialize()), allowing us to ensure that default state is generated for test modes not present in persistedData. This implementation is based on AssessmentStore, which is the only other store that overrides this method. It pulls initial data generation logic into a separate class to facilitate testing.

Motivation

This PR addresses an issue seen in Canary where adhoc visualizations are nonfunctional and the service worker logs the following error:
Error while processing browser RuntimeOnMessage event: TypeError: Cannot read properties of undefined (reading 'keyboardInteractionAssessment')

Pull request checklist

• [n/a] Addresses an existing issue: #0000
• Ran yarn null:autoadd
• Ran yarn fastpass
• Added/updated relevant unit test(s) (and ran yarn test)
• Verified code coverage for the changes made. Check coverage report at: <rootDir>/test-results/unit/coverage
• PR title AND final merge commit title both start with a semantic tag (fix:, chore:, feat(feature-name):, refactor:). See CONTRIBUTING.md.
• [n/a] (UI changes only) Added screenshots/GIFs to description above
• [n/a] (UI changes only) Verified usability with NVDA/JAWS

[dbjorge]

The restructuring into the separate initializer class LGTM, left a few comments on the initializer merging algorithm