refactor: Enhance error handling in Gemini request conversion

- Updated `CovertGemini2OpenAI` function to return an error alongside the GeminiChatRequest, improving error reporting for image processing.
- Modified `ConvertRequest` methods in both `adaptor.go` files to handle potential errors from the Gemini conversion, ensuring robust request handling.
- Improved clarity and maintainability of the code by explicitly managing error cases during request conversion.

relay/channel/gemini/adaptor.go

@@ -57,7 +57,11 @@ func (a *Adaptor) ConvertRequest(c *gin.Context, info *relaycommon.RelayInfo, re
 	if request == nil {
 		return nil, errors.New("request is nil")
 	}
-	return CovertGemini2OpenAI(*request), nil
+	ai, err := CovertGemini2OpenAI(*request)
+	if err != nil {
+		return nil, err
+	}
+	return ai, nil
 }
 
 func (a *Adaptor) ConvertRerankRequest(c *gin.Context, relayMode int, request dto.RerankRequest) (any, error) {