fix: Implement AWS handler with usage tracking and error handling

- Implemented streaming response handling for AWS handler
- Set response content type to text/event-stream
- Added error handling for failed marshaling/unmarshaling
- Updated return values to include `relaymodel.ErrorWithStatusCode` and `relaymodel.Usage`
- Improved error handling and response formatting for AWS adaptor

relay/adaptor/aws/main.go

@@ -183,6 +183,7 @@ func StreamHandler(c *gin.Context, resp *http.Response) (*relaymodel.ErrorWithSt
 	stream := awsResp.GetStream()
 	defer stream.Close()
 
+	c.Writer.Header().Set("Content-Type", "text/event-stream")
 	var usage relaymodel.Usage
 	var id string
 	c.Stream(func(w io.Writer) bool {