feat: add MiniMax image generation provider support

Add MiniMaxImageGenerationClient with support for:
- Text-to-image generation via MiniMax image-01 model
- Reference image support (subject_reference)
- Aspect ratio selection
- Proper error handling aligned with existing providers

Wire up MiniMax provider config in ImageGenerationTool, gateway,
serve, and Nanobot class.

nanobot/nanobot.py

@@ -66,6 +66,7 @@ class Nanobot:
             image_generation_provider_configs={
                 "openrouter": config.providers.openrouter,
                 "aihubmix": config.providers.aihubmix,
+                "minimax": config.providers.minimax,
             },
         )
         return cls(loop)