feat: add MiniMax provider support

[octo-patch]

Reason: Add MiniMax models to the existing provider registry.

Changes

• register a dedicated MiniMax provider through the compatible Agent SDK transport
• add MiniMax-M3 and MiniMax-M2.7 to model selection
• retain global and China endpoint configurations for both supported API transports
• expose provider selection through the CLI and document environment configuration
• add focused provider registry and endpoint tests

Checks

• npm test
• node --check on changed JavaScript files
• git diff --check
• repository diff secret scan

[octo-patch]

I corrected the Anthropic-compatible base URLs so the Agent SDK appends /v1/messages exactly once, and passed the MiniMax configuration through both the CLI and gateway entry points. I ran npm test, syntax checks, git diff --check, and a local request-path capture.