fix(anthropic): remove unsupported structured-outputs beta header for Vertex AI (#462)

Vertex AI's Anthropic integration does not support the structured-outputs-2025-11-13
beta header, causing invalid_request_error responses. Remove this header while keeping
web-search-2025-03-05 which is supported.

Author: yamitzky

src/ax/ai/anthropic/api.ts

@@ -878,8 +878,7 @@ 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',
+        'anthropic-beta': 'web-search-2025-03-05',
       });
     } else {
       if (!apiKey) {