Go: add LLM usage (#17049)

### Summary

```
RAGFlow(api/default)> CHAT WITH 'glm-4-flash@new_test@zhipu-ai' MESSAGE '30 words describes LLM';
Answer: Hello! I'm ChatGLM, an AI assistant. Feel free to ask me any questions or request help with any tasks.
Input tokens: 5
Output tokens: 28
Time: 12.748241
```

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

internal/handler/providers.go

@@ -820,6 +820,7 @@ func (h *ProviderHandler) ChatToModel(c *gin.Context) {
 		"code":              0,
 		"reasoning_content": response.ReasonContent,
 		"answer":            response.Answer,
+		"usage":             response.Usage,
 	})
 }