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/serper.json

@@ -0,0 +1,19 @@
+{
+  "id": "serper",
+  "name": "Serper",
+  "capability": "web_search",
+  "enabled": false,
+  "builtin": true,
+  "priority": 20,
+  "endpoint": "https://google.serper.dev/search",
+  "method": "POST",
+  "auth": { "type": "header", "headerName": "X-API-KEY", "apiKeyRef": "serper" },
+  "request": {
+    "bodyTemplate": { "q": "${query}", "num": "${maxResults}" }
+  },
+  "response": {
+    "resultsPath": "organic",
+    "item": { "titlePath": "title", "urlPath": "link", "snippetPath": "snippet" }
+  },
+  "timeoutMs": 10000
+}