feat(anthropic): support for user_id through metadata

[psinha40898]

The metadata field could reasonably be expanded by anthropic to include more fields, but for now it supports user_id

An external identifier for the user who is associated with the request.

This should be a uuid, hash value, or other opaque identifier. Anthropic may use this id to help detect abuse. Do not include any identifying information such as name, email address, or phone number.

maxLength256

https://platform.claude.com/docs/en/api/messages#metadata

Background

Summary

Manual Verification

Checklist

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• I have reviewed this pull request (self-review)

Future Work

anthropic may add more fields to the metadata object

Related Issues

Fixes #12386