fix: handle error message field in structure deserializer

[RanVaknin]

fixes: #2859

Error messages from AWS services can be returned with inconsistent casing (message or Message).
This change updates the error deserialization logic to handle case-insensitive message field matching.

Driver code setup:

This code uses a profile configuration that would assume a restricted role that does not have permissions to call ecs:CreateService. This will trigger an AccessDeniedException in the server's response that includes a Message field which breaks deserialization since ECS models message (lower case). The result is that the message is omitted from the error.

package main

import (
	"context"
	"fmt"
	"github.com/aws/aws-sdk-go-v2/aws"
	"github.com/aws/aws-sdk-go-v2/config"
	"github.com/aws/aws-sdk-go-v2/service/ecs"
	"log"
)

func main() {
	cfg, err := config.LoadDefaultConfig(
		context.TODO(),
		config.WithRegion("us-east-1"),
		config.WithSharedConfigProfile("restricted_role_profile"),
	)
	if err != nil {
		log.Fatalf("unable to load SDK config, %v", err)
	}

	client := ecs.NewFromConfig(cfg)

	out, err := client.CreateService(context.Background(), &ecs.CreateServiceInput{
		ServiceName:    aws.String("foo"),
		TaskDefinition: aws.String("arn:aws:ecs:us-east-1:REDACTED:task-definition/my-test-task:1"),
	})

	if err != nil {
		panic(err)
	}

	fmt.Println(out.Service.ServiceName)
}

Outputs:

panic: operation error ECS: CreateService, https response error StatusCode: 400, RequestID: REDACTED, AccessDeniedException: 

Output after change:

panic: operation error ECS: CreateService, https response error StatusCode: 400, RequestID: REDACTED, AccessDeniedException: User: arn:aws:sts::REDACTED:assumed-role/restricted_role/aws-go-sdk-1730854295921846000 is not authorized to perform: ecs:CreateService on resource: arn:aws:ecs:us-east-1:REDACTED:service/default/foo because no identity-based policy allows the ecs:CreateService action

[lucix-aws]

I can understand why you might have gone about it this way, but you've pushed complexity into the runtime that should really be handled in codegen.

The real (generated) code change that needs to happen here is less complicated than what you have currently. Basically, if we had

case "message":

before, we should now just be able to have

case "message", "Message":

That's going to be of significantly less runtime complexity than what you have currently.