refactor: merge coder-model and qwen3.5-plus, remove vision model switching

- Merge coder-model and qwen3.5-plus into a single coder-model with vision capability
- Remove vlmSwitchMode CLI argument and experimental.vlmSwitchMode setting
- Remove useVisionAutoSwitch hook and inline image format checking into useGeminiStream
- Remove ModelSwitchDialog and related vision switch UI components
- Update all related tests to reflect the simplified model structure
- Set DEFAULT_QWEN_MODEL to coder-model

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/cli/src/config/settingsSchema.ts

@@ -1196,16 +1196,6 @@ const SETTINGS_SCHEMA = {
           'Enable vision model support and auto-switching functionality. When disabled, vision models like qwen-vl-max-latest will be hidden and auto-switching will not occur.',
         showInDialog: false,
       },
-      vlmSwitchMode: {
-        type: 'string',
-        label: 'VLM Switch Mode',
-        category: 'Experimental',
-        requiresRestart: false,
-        default: undefined as string | undefined,
-        description:
-          'Default behavior when images are detected in input. Values: once (one-time switch), session (switch for entire session), persist (continue with current model). If not set, user will be prompted each time. This is a temporary experimental feature.',
-        showInDialog: false,
-      },
     },
   },
 } as const satisfies SettingsSchema;