…vercel#11857)

## Background
Anthropic’s `web_fetch` tool can return typed error results (e.g.,
`web_fetch_tool_result_error`). These were causing a crash during prompt
serialization because `convertToAnthropicMessagesPrompt` attempted
`JSON.parse` on an object value, producing `"[object Object]" is not
valid JSON`.

## Summary
- Handle `web_fetch` error results when `output.value` is either a
string or an object, defaulting `error_code` to `'unknown'` on malformed
JSON.
- Added tests covering object-form and malformed-string error values.
- Added a patch changeset for `@ai-sdk/anthropic`.

## Manual Verification
- Ran package tests: `pnpm test:node` and `pnpm test:edge` in
`packages/anthropic`.
- Confirmed new web_fetch error-handling tests pass.

## Checklist
- [x] Tests have been added / updated (for bug fixes / features)
- [ ] Documentation has been added / updated (for bug fixes / features)
- [x] A _patch_ changeset for relevant packages has been added (for bug
fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)

## Future Work
- None.

## Related Issues
- Fixes vercel#11856

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Co-authored-by: Aayush Kapoor <83492835+aayush-kapoor@users.noreply.github.com>