feat: add MiniMax as native provider

[ricorna]

Depends on #24 — this PR includes those commits and adds the MiniMax provider on top.

Motivation

MiniMax offers competitive models (MiniMax M2.5) with both OpenAI-compatible and Anthropic-compatible API formats. MiniMax has the most variation of any provider we support:

• Region: International (api.minimax.io) vs China mainland (api.minimaxi.com)
• API format: OpenAI-compatible (/v1) vs Anthropic-compatible (/anthropic)
• Plan: Standard pay-per-token API key vs Coding Plan (sk-cp- prefix, separate key, subscription billing)

This is a good stress test of the base_url infrastructure from #24 — it handles all three dimensions cleanly. Users select their endpoint preset (or enter a custom URL) and provide their key. No code changes needed for any combination.

Thanks to @ZerGo0 whose research in #13 identified MiniMax's coding plan endpoint structure.

Changes

• Added minimax_key / MINIMAX_API_KEY config and env var
• MiniMax dispatch via call_anthropic_compatible (Anthropic message format)
• Default endpoint: api.minimax.io/v1/chat/completions
• Endpoint presets: "MiniMax (International)" / "MiniMax (China)" / "MiniMax Anthropic" / "MiniMax Coding Plan"
• Provider card with icon, status endpoint, minimax/ model prefix
• Default routing config

Breaking Changes

None. New provider — no existing behavior affected.

How to Test

# Standard MiniMax (international, OpenAI-compatible)
MINIMAX_API_KEY="sk-..." cargo run

# MiniMax Coding Plan (Anthropic-compatible)
MINIMAX_API_KEY="sk-cp-..." \
MINIMAX_BASE_URL="https://api.minimax.io/anthropic/v1/messages" \
cargo run

# China mainland
MINIMAX_API_KEY="sk-..." \
MINIMAX_BASE_URL="https://api.minimaxi.com/v1/chat/completions" \
cargo run

# Or via web UI → Settings → Providers → MiniMax → Endpoint dropdown

Verified

Tested with a MiniMax Coding Plan subscription (sk-cp- key) against the Anthropic-compatible endpoint (api.minimax.io/anthropic). Chat completions return successfully. Using the Coding Plan key against the standard pay-per-token endpoint returns the expected error — confirming the sk-cp- key is scoped to the Coding Plan surface. Endpoint preset selector in the web UI correctly offers International, China, Anthropic, and Coding Plan options.

Diff stats

9 files changed, 109 insertions(+), 3 deletions(-)

[hotzen]

hey, any ETA on this getting merged?
eagerly awaited, thanks!

[ricorna]

hey, any ETA on this getting merged?

eagerly awaited, thanks!

This relies on #24 being merged first (this part is trivial on top of it)

So as soon as there's any movement on that, this probably goes in soon after.

But it's out of my hands for the moment.

[Marenz]

I am using this PR with some of my own (unrelated to this topic) fixes, so LGTM. Though it needs a rebase.