fix: validate and apply the ListBuckets bucket-region query

ListBuckets ignored bucket-region: a value that is not a region was
accepted, and a region other than the gateway one still listed every
bucket. S3 rejects an invalid value with 400 InvalidArgument, "Argument
value abc is not a valid AWS Region", and lists only the buckets of the
requested region.

Check bucket-region against the AWS region code format, always
accepting the gateway region whatever its name, and return an empty
list for any other region, since every bucket lives in the gateway
region. Enable the REST tests that cover both cases.

Fixes #1923
Fixes #1930

tests/integration/ListBuckets.go

@@ -17,6 +17,7 @@ package integration
 import (
 	"context"
 	"fmt"
+	"slices"
 	"sort"
 
 	"github.com/aws/aws-sdk-go-v2/service/s3"
@@ -271,6 +272,60 @@ func ListBuckets_invalid_max_buckets(s *S3Conf) error {
 	})
 }
 
+func ListBuckets_invalid_bucket_region(s *S3Conf) error {
+	testName := "ListBuckets_invalid_bucket_region"
+	return actionHandler(s, testName, func(s3client *s3.Client, bucket string) error {
+		ctx, cancel := context.WithTimeout(context.Background(), shortTimeout)
+		_, err := s3client.ListBuckets(ctx, &s3.ListBucketsInput{
+			BucketRegion: getPtr("abc"),
+		})
+		cancel()
+		return checkApiErr(err, s3err.GetInvalidArgBucketRegion("abc"))
+	})
+}
+
+func ListBuckets_bucket_region(s *S3Conf) error {
+	testName := "ListBuckets_bucket_region"
+	return actionHandler(s, testName, func(s3client *s3.Client, bucket string) error {
+		listBuckets := func(region string) ([]types.Bucket, error) {
+			ctx, cancel := context.WithTimeout(context.Background(), shortTimeout)
+			out, err := s3client.ListBuckets(ctx, &s3.ListBucketsInput{
+				BucketRegion: &region,
+			})
+			cancel()
+			if err != nil {
+				return nil, err
+			}
+			return out.Buckets, nil
+		}
+
+		buckets, err := listBuckets(s.awsRegion)
+		if err != nil {
+			return err
+		}
+		if !slices.ContainsFunc(buckets, func(b types.Bucket) bool { return getString(b.Name) == bucket }) {
+			return fmt.Errorf("expected bucket %v in the %v listing, instead got %v",
+				bucket, s.awsRegion, buckets)
+		}
+
+		// every bucket of the gateway lives in its own region
+		otherRegion := "us-west-2"
+		if s.awsRegion == otherRegion {
+			otherRegion = "us-east-1"
+		}
+		buckets, err = listBuckets(otherRegion)
+		if err != nil {
+			return err
+		}
+		if len(buckets) != 0 {
+			return fmt.Errorf("expected an empty %v listing, instead got %v",
+				otherRegion, buckets)
+		}
+
+		return nil
+	})
+}
+
 func ListBuckets_truncated(s *S3Conf) error {
 	testName := "ListBuckets_truncated"
 	return actionHandler(s, testName, func(s3client *s3.Client, bucket string) error {