refactor: Migrate AI relay APIs and add audit logging

- Handle all the cases in `relayMode` and `apiType` switch in a consistent manner.
- Retire AI Proxy Library API call pattern.
- Add new cases (Zhipu, Ali, Tencent, Gemini) to generalize the conversion and handling of requests.
- Log the requests if conservation audit is enabled.

controller/relay.go

@@ -328,6 +328,7 @@ type TextResponse struct {
 	Choices []OpenAITextResponseChoice `json:"choices"`
 	Usage   `json:"usage"`
 	Error   OpenAIError `json:"error"`
+	Content string      `json:"-"`
 }
 
 type OpenAITextResponseChoice struct {