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/image-understanding/minimax.json

@@ -0,0 +1,18 @@
+{
+  "id": "minimax",
+  "name": "MiniMax 图像理解（内测网关）",
+  "capability": "image_understanding",
+  "enabled": false,
+  "builtin": true,
+  "priority": 30,
+  "endpoint": "${baseUrl}/understand_image",
+  "method": "POST",
+  "baseUrlRef": "internal-testing",
+  "auth": { "type": "bearer" },
+  "request": {
+    "imageMode": "url",
+    "bodyTemplate": { "prompt": "${prompt}", "image_url": "${imageUrl}" }
+  },
+  "response": { "textPath": "text" },
+  "timeoutMs": 30000
+}