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/cli/commands.py

@@ -642,6 +642,7 @@ def serve(
             image_generation_provider_configs={
                 "openrouter": runtime_config.providers.openrouter,
                 "aihubmix": runtime_config.providers.aihubmix,
+                "minimax": runtime_config.providers.minimax,
             },
         )
     except ValueError as exc:
@@ -755,6 +756,7 @@ def _run_gateway(
         image_generation_provider_configs={
             "openrouter": config.providers.openrouter,
             "aihubmix": config.providers.aihubmix,
+            "minimax": config.providers.minimax,
         },
         provider_snapshot_loader=load_provider_snapshot,
         runtime_model_publisher=lambda model, preset: publish_runtime_model_update(