Add MiniMax image generation fallback

[vshen009]

Summary

• add provider-aware image size normalization so MiniMax and OpenAI can share the same generate flow
• probe OpenAI first and fall back cleanly to MiniMax when available
• add a focused regression test for provider size conversion defaults and mappings

Testing

• bun run build
• bun test
• bun test test/design-generate.test.ts