feat(api-providers): 新增 web_search / image_understanding 声明式后端默认配置 (#31)

- api-providers/{web-search,image-understanding}/*.json + _index.json
- 新增 schemas/api-provider.schema.json，validate.mjs 纳入校验（CI 自检）
- 不改 compute/ 与 manifest，presetDataVersion 不变：老客户端忽略新目录，线上不受影响

Co-authored-by: yi-ge <jackyoncode@gmail.com>

api-providers/web-search/_index.json

@@ -0,0 +1,4 @@
+{
+  "description": "web_search 工具后端加载顺序索引",
+  "order": ["tavily", "serper", "minimax"]
+}