fix: 修正 conversion 代理路径和错误边界

backend/internal/conversion/engine_test.go

@@ -2,6 +2,7 @@ package conversion
 
 import (
 	"encoding/json"
+	"strings"
 	"testing"
 
 	"nex/backend/internal/conversion/canonical"
@@ -498,12 +499,13 @@ func TestCreateStreamConverter_ModelOverride_SmartPassthrough(t *testing.T) {
 	_, ok := converter.(*SmartPassthroughStreamConverter)
 	assert.True(t, ok)
 
-	// 验证 chunk 改写
-	chunks := converter.ProcessChunk([]byte(`{"model":"gpt-4","choices":[]}`))
+	// 验证 SSE frame 中的 data JSON 被改写
+	chunks := converter.ProcessChunk([]byte(`data: {"model":"gpt-4","choices":[]}` + "\n\n"))
 	require.Len(t, chunks, 1)
 
 	var resp map[string]interface{}
-	require.NoError(t, json.Unmarshal(chunks[0], &resp))
+	payload := strings.TrimPrefix(strings.TrimSpace(string(chunks[0])), "data: ")
+	require.NoError(t, json.Unmarshal([]byte(payload), &resp))
 	assert.Equal(t, "openai/gpt-4", resp["model"])
 }