fix(auth): use bearer auth for MiniMax Anthropic endpoints (#4028)
MiniMax's /anthropic endpoints implement Anthropic's Messages API but require Authorization: Bearer instead of x-api-key. Without this fix, MiniMax users get 401 errors in gateway sessions. Adds _requires_bearer_auth() to detect MiniMax endpoints and route through auth_token in the Anthropic SDK. Check runs before OAuth token detection so MiniMax keys aren't misclassified as setup tokens. Co-authored-by: kshitijk4poor <kshitijk4poor@users.noreply.github.com>
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