fix(addons): improve addon validation [Dev-only]

[david-allison]

Note

Assisted-by: Claude Fable 5

Purpose / Description

• A number of addon validation mechanisms were inconsistent: throwing rather than returning.
• We are now in Kotlin, so a sealed class makes sense
• I want to move to a model where a state store defines each addon, so the individual keys for shared preferences are removed

How Has This Been Tested?

Test-only

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner