fix: use bearer auth for MiniMax in Claude Code

[kapelame]

What & why

MiniMax preset models do not set auth_scheme, so Claude Code received the credential through ANTHROPIC_API_KEY and MiniMax rejected the request with HTTP 401. MiniMax's Claude Code setup uses ANTHROPIC_AUTH_TOKEN and Bearer authentication.

How

When auth_scheme is unset, default the minimax-cn and minimax-en providers to Bearer authentication in the Claude Code model injector. Explicit auth-scheme values and all other providers keep their existing behavior. Regression coverage exercises both MiniMax regions.

Verification

• make check
• Relevant E2E target (if any): no dedicated MiniMax E2E target exists
• Manual smoke: local Parsar dispatched Claude Code to a paired development machine and no longer received the authentication 401; the upstream service subsequently returned a transient 503

Notes for reviewers

The change is limited to the Claude Code runtime injector; direct Anthropic-compatible API health checks keep their existing authentication behavior.

[kapelame]

Closing this PR because it was opened under a mistaken scope. No merge is intended.