feat: add MiniMax as first-class LLM provider

[octo-patch]

Summary

Add MiniMax M2.7 and M2.7-highspeed as first-class LLM provider options in PaperDebugger. MiniMax offers 1M context window and 128K output at competitive pricing through an OpenAI-compatible API.

Changes

• Model registry: Add MiniMax M2.7 ($1.00/$4.00 per 1M tokens) and M2.7-highspeed ($0.50/$2.00) to allModels with dual-slug support (OpenRouter + direct API)
• Provider routing: Route MiniMax models to MiniMax API when MINIMAX_API_KEY is configured; falls back to OpenRouter/inference endpoint otherwise
• Configuration: Add MINIMAX_API_KEY and MINIMAX_BASE_URL environment variables (default: https://api.minimax.io/v1)
• Temperature clamping: Enforce MiniMax temperature range [0.0, 1.0]
• Documentation: Update README.md and DEVELOPMENT.md with MiniMax provider info

How it works

1. Via OpenRouter (default): MiniMax models work out of the box through the existing inference/OpenRouter endpoint - no additional configuration needed
2. Via direct MiniMax API (optional): Set MINIMAX_API_KEY env var to route MiniMax models directly to api.minimax.io/v1 for potentially lower latency and cost

Files changed (14 files, 661 additions)

File	Change
internal/api/chat/list_supported_models_v2.go	Add MiniMax models + slug selection logic
internal/services/toolkit/client/client_v2.go	MiniMax endpoint routing in GetOpenAIClient()
internal/services/toolkit/client/completion_v2.go	Set model name for provider-aware routing
internal/services/toolkit/client/utils_v2.go	Temperature clamping for MiniMax
internal/libs/cfg/cfg.go	Add MiniMax config fields
internal/models/llm_provider.go	Add IsMiniMaxModel() helper
README.md	Mention MiniMax in architecture overview
docs/DEVELOPMENT.md	Document MiniMax env vars

Test plan

• 15 unit tests: model detection, config, registry validation, slug uniqueness, pricing
• 12 unit tests: temperature clamping, tools support, routing logic, params
• 4 integration tests: provider routing with/without server key, user key scenarios
• go build ./... passes
• All existing tests still pass