feat: SimplifyAnthropicRequest

- Remove `N` parameter from Anthropic request
- Append system messages from Claude to the `System` field instead of sending them as separate messages

relay/channel/anthropic/main.go

@@ -47,6 +47,7 @@ func ConvertRequest(textRequest model.GeneralOpenAIRequest) *Request {
 	}
 	claudeRequest.Messages = filteredMessages
 
+	claudeRequest.N = 0 // anthropic's messages API not support n
 	return &claudeRequest
 }