… metadata

In `_get_metadata_from_response`, the `CompletionUsage` object was wrapped in
a single-element tuple due to a trailing comma after the closing parenthesis:

    metadata["usage"] = (
        CompletionUsage(...),   # <-- trailing comma made this a tuple
    )

This caused `metadata["usage"]` to be a `tuple` instead of a `CompletionUsage`
instance, breaking any code that expected to access token counts via
`metadata["usage"].prompt_tokens` or `metadata["usage"].completion_tokens`.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>