Replaces the previous placeholder for default LLM selection with a working default AI model selector in settings. Updates SettingsModel and related providers to support storing and updating the default AI model as a JSON object. Integrates the selector UI and ensures new AI requests use the default model if set.
Refactored collection state management to handle API type changes and AI request models. Updated widgets and tests to support nullable HTTP methods and AI request models, and improved response body rendering for AI responses.
