xAI: route x_search through public api seam

extensions/xai/api.ts

@@ -14,6 +14,15 @@ export {
   XAI_DEFAULT_MAX_TOKENS,
 } from "./model-definitions.js";
 export { isModernXaiModel, resolveXaiForwardCompatModel } from "./provider-models.js";
+export {
+  __testing as xSearchTesting,
+  buildXaiXSearchPayload,
+  requestXaiXSearch,
+  resolveXaiXSearchInlineCitations,
+  resolveXaiXSearchMaxTurns,
+  resolveXaiXSearchModel,
+  type XaiXSearchOptions,
+} from "./src/x-search-shared.js";
 import { normalizeXaiModelId } from "./model-id.js";
 export { normalizeXaiModelId };