Python: fix(anthropic): don't mutate caller metadata when mapping user to user_id

[Anish Mehta (anishmehta24)]

Motivation & Context

When user is set, AnthropicClient._prepare_options maps it to metadata.user_id, but it writes user_id straight into the metadata dict the caller passed in. If the same metadata dict is reused across requests (a shared default, a per-session trace dict, etc.), the first user sticks: the "user_id" not in metadata check sees the leaked value on the next call and every later request goes out attributed to the first user.

client = AnthropicClient(api_key="x", model="claude-sonnet-4-5")
meta = {"trace": "t1"}
msgs = [Message(role="user", contents=[Content.from_text("hi")])]
client._prepare_options(msgs, {"user": "alice", "metadata": meta})["metadata"]
# {'trace': 't1', 'user_id': 'alice'}
meta
# {'trace': 't1', 'user_id': 'alice'}   <- caller's dict was changed
client._prepare_options(msgs, {"user": "bob", "metadata": meta})["metadata"]
# {'trace': 't1', 'user_id': 'alice'}   <- bob's request is sent as alice

merge_chat_options only shallow-copies the top-level options, so the nested metadata dict passed to get_response is the same object that reaches _prepare_options.

Description & Review Guide

• What are the major changes? The user -> metadata.user_id mapping now works on a copy of the metadata dict instead of the caller's one.
• What is the impact of these changes? The caller's metadata is left untouched, and each request gets the user_id for its own user. An explicit user_id already in metadata still wins, as before.
• What do you want reviewers to focus on? Nothing in particular, it's a one-line change.

Related Issue

No existing issue.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

Test: test_prepare_options_user_does_not_mutate_caller_metadata in packages/anthropic/tests/test_anthropic_client.py; it fails on main. The anthropic tests pass (196 passed, 9 skipped), ruff check/format are clean, and pyright shows no new errors.

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[Eduard van Valkenburg (eavanvalkenburg)]

Anish Mehta (@anishmehta24) Reviewed the current head and found no blocking issues.