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>

Author: teknium1