Gateway assume_role checks now cover the mantle provider

What

A gateway sits between Claude Code and the model provider, and each provider it forwards requests to is called an upstream. Two configuration checks on its upstreams changed. assume_role is the setting that tells an upstream which AWS role to sign in as.

• The check that rejects assume_role combined with aws_bearer_token used to apply only to providers named bedrock. It now applies to providers named mantle too, and it now checks each entry's own provider.
• The warning for a mantle and bedrock upstream pair now works both ways. Before, it fired only when the mantle upstream was paired with a bedrock upstream that set assume_role.session_name. It now fires whenever one of the pair sets assume_role.session_name and the other sets none.
• The reworded message tells you to set assume_role (role_arn and session_name) on the other upstream too.

Why

If you run a gateway with mantle or bedrock upstreams, mismatched or conflicting authentication settings are now caught in more cases. The new warning also tells you exactly what to add.