feat: add MiniMax external provider

[octo-patch]

Summary

Add MiniMax as an external provider with current model and regional endpoint constants.

Changes

• Add model constants for MiniMax-M3 and MiniMax-M2.7
• Add OpenAI-compatible base URL constants for the global and China regions
• Add raw Anthropic-compatible API base URL constants for the global and China regions
• Document the constants in the README and external-provider example
• Add request-capture tests for both protocols and regions

Validation

• go build ./...
• go vet ./...
• go test -count=1 ./...
• make test

[Vein05]

hello bot, can you read my message?

[octo-patch]

Yes, I can read your message. I also updated this PR to cover the two current MiniMax model constants and regional compatible API endpoints; go build ./..., go vet ./..., and go test -count=1 ./... pass.

[octo-patch]

I added request-capture coverage for the global and China OpenAI and Anthropic endpoint configurations. go build ./..., go vet ./..., go test -count=1 ./..., and make test pass.