feat(providers): add MiniMax regional presets

[octo-patch]

Reason: Add built-in MiniMax configuration for current models and regional endpoints.

Summary

• resolve minimax and minimax-cn entries through the existing provider contract
• include the current model catalog, capabilities, and estimated token pricing
• preserve explicit entry overrides and report built-in endpoint provenance

Checks

• npx vitest run tests/unit/provider-contract.test.ts tests/unit/cost.test.ts
• npm run typecheck
• npm run build
• npx vitest run tests/integration/provider-contract.test.ts