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/ui/commands/modelCommand.ts

@@ -11,7 +11,6 @@ import type {
   MessageActionReturn,
 } from './types.js';
 import { CommandKind } from './types.js';
-import { getAvailableModelsForAuthType } from '../models/availableModels.js';
 import { t } from '../../i18n/index.js';
 
 export const modelCommand: SlashCommand = {
@@ -52,22 +51,6 @@ export const modelCommand: SlashCommand = {
       };
     }
 
-    const availableModels = getAvailableModelsForAuthType(authType);
-
-    if (availableModels.length === 0) {
-      return {
-        type: 'message',
-        messageType: 'error',
-        content: t(
-          'No models available for the current authentication type ({{authType}}).',
-          {
-            authType,
-          },
-        ),
-      };
-    }
-
-    // Trigger model selection dialog
     return {
       type: 'dialog',
       dialog: 'model',