feat(ai): add MiniMax provider

[octo-patch]

Reason: Add a first-class MiniMax provider with base-URL credentials, live model discovery, and admin UI support.

Changed:

• Added the MiniMax driver and registered it in the AI provider registry.
• Extended the admin provider catalog and credential forms for MiniMax base-URL setup.
• Added provider model lookup coverage and a MiniMax-specific driver test.
• Updated the providers tab test to cover the MiniMax setup flow.

Checks:

• bun test server/ai/drivers/minimax.test.ts src/__tests__/ai/providersTab.test.tsx
• bun run build