fix(anthropic): add anthropic-beta header for web-search on Vertex AI (#457)

* fix(anthropic): add anthropic-beta header for web-search on Vertex AI

* Update 'anthropic-beta' header value

---------

Co-authored-by: Spacy <832235+dosco@users.noreply.github.com>

Author: yamitzky

src/ax/ai/anthropic/api.ts

@@ -884,6 +884,7 @@ export class AxAIAnthropic<TModelKey = string> extends AxBaseAI<
       apiURL = `https://${tld}.googleapis.com/v1/projects/${projectId}/locations/${region}/publishers/anthropic/`;
       headers = async () => ({
         Authorization: `Bearer ${await apiKey()}`,
+        'anthropic-beta': 'structured-outputs-2025-11-13, web-search-2025-03-05',
       });
     } else {
       if (!apiKey) {