refactor: update authentication handling and model configuration

- Enhanced authentication method validation in `auth.ts` and `auth.test.ts`.
- Introduced new model provider configuration logic
- Updated environment variable handling for various auth types.
- Removed deprecated utility functions and tests related to fallback mechanisms.

packages/cli/src/config/settingsSchema.ts

@@ -113,7 +113,7 @@ const SETTINGS_SCHEMA = {
     description:
       'Model providers configuration grouped by authType. Each authType contains an array of model configurations.',
     showInDialog: false,
-    mergeStrategy: MergeStrategy.SHALLOW_MERGE,
+    mergeStrategy: MergeStrategy.REPLACE,
   },
 
   general: {