Add MiniMax provider to provider management UI

[octo-patch]

Reason: Add MiniMax as a first-class OpenAI-compatible provider with its default endpoint and current model.

This change adds MiniMax to the provider selector, applies its global endpoint and default model, and treats it as an OpenAI-compatible endpoint for model probing and saved-provider forms. It also adds the MiniMax provider label while preserving the existing provider naming coverage.

Checks:

• zig build test -Dembed-ui=false
• npm run build (in ui)
• git diff --check
• Secret scan

[octo-patch]

Updated the MiniMax presets to include the global and China OpenAI-compatible endpoints and the current MiniMax-M3 and MiniMax-M2.7 model choices, and pushed commit 1ea2fe6. I ran npm --prefix ui run build and bash tests/test_backend.sh; all four CI jobs pass.