[feat] Implement AssetService behind settings flag (#5404)

* [feat] add Comfy.Assets.UseAssetAPI to CORE_SETTINGS

* [feat] create AssetService

1. Add service for accessing new Asset API
2. Add fallback model paths logic so empty model directories appear for
   the user.
3. Copious tests for them all.

Co-Authored-By: Claude <noreply@anthropic.com>

* [feat] switch between assets and file paths for model data

* [feat] ignore assets with "missing" tag

* [fix] formatting and style

* [fix] call assets API with the correct filters

* [feat] elminate unused modelPath code

* [fix] remove stray comment

* [fix] model manager api was not parsed correctly

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/constants/coreSettings.ts

@@ -972,5 +972,14 @@ export const CORE_SETTINGS: SettingParams[] = [
     defaultValue: false,
     experimental: true,
     versionAdded: '1.27.1'
+  },
+  {
+    id: 'Comfy.Assets.UseAssetAPI',
+    name: 'Use Asset API for model library',
+    type: 'boolean',
+    tooltip:
+      'Use new asset API instead of experiment endpoints for model browsing',
+    defaultValue: false,
+    experimental: true
   }
 ]