feat: add MiniMax provider registry support

[octo-patch]

Reason: add target provider/model to existing provider registry

Adds MiniMax M3 and MiniMax M2.7 to the embedded registry with model metadata, global and China endpoints, and completions-compatible and messages-compatible adapters. Keeps the messages-compatible base URL unchanged so the SDK appends /v1/messages.

Checks:

• go test -race ./internal/provider ./internal/config
• go vet ./internal/provider ./internal/config
• golangci-lint run --timeout=5m ./internal/provider/... ./internal/config/...

[octo-patch]

Thanks for the review. I corrected the MiniMax-M3 registry pricing (input 0.6, output 2.4, cache-read 0.12 per million tokens) to match the provider spec and added price assertions to the registry test. I ran go vet, go test -race ./internal/provider/... ./internal/config/..., and go build ./... — all green.