feat: upgrade MiniMax default model to M3

[octo-patch]

Summary

Add MiniMax provider to PentAGI with MiniMax-M3 as the default model, keeping MiniMax-M2.7 and MiniMax-M2.7-highspeed as alternatives.

Changes

• Add new MiniMax provider package (backend/pkg/providers/minimax/) implementing the provider.Provider interface via the OpenAI-compatible https://api.minimax.io/v1 endpoint.
• Set MiniMax-M3 as the default model for all agent types (simple, primary_agent, assistant, generator, refiner, adviser, reflector, searcher, enricher, coder, installer, pentester).
• Keep MiniMax-M2.7 and MiniMax-M2.7-highspeed in the model list as alternatives.
• Register ProviderMiniMax type, DefaultProviderNameMiniMax name, and wire the provider into NewProviderController / NewProvider / buildProviderFromConfig / buildProviderFromConfig / GetProvider paths.
• Add env vars MINIMAX_API_KEY, MINIMAX_SERVER_URL (default https://api.minimax.io/v1), MINIMAX_PROVIDER and the matching secret pattern.
• Add provider.ProviderMiniMax to the Valid() whitelist in pkg/server/models/providers.go (required for REST API acceptance).
• Unit tests covering config loading, default-model sanity, models list (M3 first, no legacy M1/M2/M2.1/M2.5), model prefix behaviour, and missing-API-key error.

Why

MiniMax-M3 is MiniMax's latest flagship model (512K context, 128K max output, image input support) and is the right default for a security-testing workload that benefits from large context and multimodal input. MiniMax-M2.7 and its high-speed variant are retained for users who want to pin to the previous generation.

Model list

• MiniMax-M3 (default) — 512K context, 128K output, image input, input $0.60 / output $2.40 per 1M tokens
• MiniMax-M2.7 — input $0.40 / output $1.10
• MiniMax-M2.7-highspeed — input $0.40 / output $1.10

No legacy models (MiniMax-M1 / MiniMax-M2 / MiniMax-M2.1 / MiniMax-M2.5) are included.

API docs

• OpenAI-compatible Chat: https://platform.minimax.io/docs/api-reference/text-openai-api
• Pricing: https://platform.minimax.io/docs/guides/pricing-paygo

Testing

• go test ./pkg/providers/minimax/... — 7/7 passing (config loading, provider type, models list with M3 as first entry, prefix behaviour, missing key, usage extraction).
• go build ./pkg/providers/minimax/... — clean.