fix(translator): handle gjson strings in OpenAI response formatting

fix(registry): add `MiniMax-M2` model to registry definitions

internal/registry/model_definitions.go

@@ -438,6 +438,7 @@ func GetIFlowModels() []*ModelInfo {
 		{ID: "qwen3-235b-a22b-thinking-2507", DisplayName: "Qwen3-235B-A22B-Thinking", Description: "Qwen3 235B A22B Thinking (2507)"},
 		{ID: "qwen3-235b-a22b-instruct", DisplayName: "Qwen3-235B-A22B-Instruct", Description: "Qwen3 235B A22B Instruct"},
 		{ID: "qwen3-235b", DisplayName: "Qwen3-235B-A22B", Description: "Qwen3 235B A22B"},
+		{ID: "minimax-m2", DisplayName: "MiniMax-M2", Description: "MiniMax M2"},
 	}
 	models := make([]*ModelInfo, 0, len(entries))
 	for _, entry := range entries {

internal/translator/openai/openai/responses/openai_openai-responses_request.go

@@ -2,6 +2,7 @@ package responses
 
 import (
 	"bytes"
+
 	"github.com/tidwall/gjson"
 	"github.com/tidwall/sjson"
 )
@@ -147,6 +148,11 @@ func ConvertOpenAIResponsesRequestToOpenAIChatCompletions(modelName string, inpu
 
 			return true
 		})
+	} else if input.Type == gjson.String {
+		msg := "{}"
+		msg, _ = sjson.Set(msg, "role", "user")
+		msg, _ = sjson.Set(msg, "content", input.String())
+		out, _ = sjson.SetRaw(out, "messages.-1", msg)
 	}
 
 	// Convert tools from responses format to chat completions format