refactor(providers): add flexible app type resolution with dual parameter support

Add `resolve_app_type` helper to support both enum and string-based app type
parameters across all provider commands. This change:

- Eliminates implicit default to Claude (previously used `unwrap_or`)
- Supports two parameter forms: `app_type` (enum, priority 1) and `app` (string, priority 2)
- Provides explicit error handling when both parameters are missing
- Updates all 14 provider command functions with consistent parameter validation
- Fixes tray menu provider switching to pass the new `app` parameter

This dual-parameter approach maintains backward compatibility while enabling
future CLI tool integration and more flexible API usage patterns.

Technical details:
- Priority order: `app_type` enum > `app` string > error
- Invalid `app` strings now return errors instead of defaulting
- All existing tests pass (45/45)

src-tauri/src/lib.rs

@@ -267,7 +267,12 @@ fn switch_provider_internal(
         let app_type_str = app_type.as_str().to_string();
         let provider_id_clone = provider_id.clone();
 
-        crate::commands::switch_provider(app_state.clone(), Some(app_type), provider_id)
+        crate::commands::switch_provider(
+            app_state.clone(),
+            Some(app_type),
+            Some(app_type_str.clone()),
+            provider_id,
+        )
             .map_err(AppError::Message)?;
 
         // 切换成功后重新创建托盘菜单