fix(dashscope-image-gen): 默认模型改为 gpt-image-2 v1.4.0 (#29)

## Summary

上游 proxy (desirecore-cloud at api.yizhioa.cn) 仅支持 `gpt-image-2`
图片生成模型，`wan2.7-image` 返回 `model_price_error`。

The upstream proxy (desirecore-cloud) only supports `gpt-image-2` for
image generation. `wan2.7-image` returns `model_price_error`.

### Changes

- Default model changed from `wan2.7-image` → `gpt-image-2`
- `wan2.7-image` / `wan2.7-image-pro` kept as optional (requires
user-configured DashScope provider)
- Added auto-fallback strategy: `model_price_error` → retry with
`gpt-image-2`
- Size values adapted for gpt-image-2: `1536x1024` / `1024x1536`
- Skill display name updated: 阿里云 文生图 → AI 文生图
- Market version: 1.2.4 → 1.2.5, skill version: 1.3.0 → 1.4.0

### Test

- `gpt-image-2` via `/images/generations` endpoint: verified working
(b64_json response, 1.5MB PNG)
- Upload to media-store: verified working
- `dc-media://` rendering: verified working

manifest.json

@@ -1,6 +1,6 @@
 {
   "name": "DesireCore Official Market",
-  "version": "1.2.4",
+  "version": "1.2.5",
   "schemaVersion": "1.1.0",
   "supportedLocales": ["zh-CN", "en-US"],
   "defaultLocale": "en-US",
@@ -28,7 +28,7 @@
   "stats": {
     "totalAgents": 1,
     "totalSkills": 25,
-    "lastUpdated": "2026-05-31"
+    "lastUpdated": "2026-06-15"
   },
   "features": [
     "verified-only",