feat: add MiniMax provider support (#1406)

Add MiniMax as a new AI model provider with OpenAI-compatible API.

Supported models:
- MiniMax-M2.5 (default) - Peak Performance, Ultimate Value
- MiniMax-M2.5-highspeed - Same performance, faster and more agile

Changes:
- Add MiniMax client (mcp/minimax_client.go) with OpenAI-compatible API
- Add comprehensive unit tests (mcp/minimax_client_test.go)
- Add WithMiniMaxConfig option (mcp/options.go)
- Register MiniMax provider in trader, debate engine, backtest, and API
- Add MiniMax to frontend provider config and model icons
- Add MiniMax SVG icon

API Base URL: https://api.minimax.io/v1

backtest/ai_client.go

@@ -71,6 +71,13 @@ func configureMCPClient(cfg BacktestConfig, base mcp.AIClient) (mcp.AIClient, er
 		oaiC := mcp.NewOpenAIClientWithOptions()
 		oaiC.(*mcp.OpenAIClient).SetAPIKey(cfg.AICfg.APIKey, cfg.AICfg.BaseURL, cfg.AICfg.Model)
 		return oaiC, nil
+	case "minimax":
+		if cfg.AICfg.APIKey == "" {
+			return nil, fmt.Errorf("minimax provider requires api key")
+		}
+		mmC := mcp.NewMiniMaxClientWithOptions()
+		mmC.(*mcp.MiniMaxClient).SetAPIKey(cfg.AICfg.APIKey, cfg.AICfg.BaseURL, cfg.AICfg.Model)
+		return mmC, nil
 	case "custom":
 		if cfg.AICfg.BaseURL == "" || cfg.AICfg.APIKey == "" || cfg.AICfg.Model == "" {
 			return nil, fmt.Errorf("custom provider requires base_url, api key and model")
@@ -125,6 +132,11 @@ func cloneBaseClient(base mcp.AIClient) *mcp.Client {
 			cp := *c.Client
 			return &cp
 		}
+	case *mcp.MiniMaxClient:
+		if c != nil && c.Client != nil {
+			cp := *c.Client
+			return &cp
+		}
 	}
 	// Fall back to a new default client
 	return mcp.NewClient().(*mcp.Client)