fix (provider/anthropic): streaming json output (#6901)

## Background

`streamObject` was not working with the Anthropic provider.

## Summary

Fix streaming with json output mode.

## Verification

Ran example.

## Future Work

* fine grained streaming beta header

Author: lgrammel

.changeset/green-olives-hope.md

@@ -0,0 +1,5 @@
+---
+'@ai-sdk/anthropic': patch
+---
+
+fix (provider/anthropic): streaming json output

examples/ai-core/src/stream-object/anthropic.ts

@@ -5,8 +5,8 @@ import { z } from 'zod/v4';
 
 async function main() {
   const result = streamObject({
-    model: anthropic('claude-3-5-sonnet-20240620'),
-    maxOutputTokens: 2000,
+    model: anthropic('claude-sonnet-4-20250514'),
+    maxOutputTokens: 5000,
     schema: z.object({
       characters: z.array(
         z.object({
@@ -20,6 +20,10 @@ async function main() {
     }),
     prompt:
       'Generate 3 character descriptions for a fantasy role playing game.',
+    onError: err => {
+      console.error(err);
+    },
+    temperature: 0,
   });
 
   for await (const partialObject of result.partialObjectStream) {