s3tables: refactor bucket name validation into single function

Combine length, character, and reserved pattern validation into validateBucketName()
which returns descriptive error messages. Keep isValidBucketName() for backward
compatibility. This simplifies handler validation and provides better error reporting.

weed/s3api/s3tables/utils.go

@@ -47,6 +47,12 @@ func parseTableFromARN(arn string) (bucketName, namespace, tableName string, err
 		return "", "", "", fmt.Errorf("invalid table ARN: %s", arn)
 	}
 
+	// Validate bucket name
+	bucketName = matches[1]
+	if err := validateBucketName(bucketName); err != nil {
+		return "", "", "", fmt.Errorf("invalid bucket name in ARN: %v", err)
+	}
+
 	// Namespace is already constrained by the regex; validate it directly.
 	namespace = matches[2]
 	_, err = validateNamespace([]string{namespace})
@@ -62,7 +68,7 @@ func parseTableFromARN(arn string) (bucketName, namespace, tableName string, err
 	if _, err := validateTableName(tableNameUnescaped); err != nil {
 		return "", "", "", fmt.Errorf("invalid table name in ARN: %v", err)
 	}
-	return matches[1], namespace, tableNameUnescaped, nil
+	return bucketName, namespace, tableNameUnescaped, nil
 }
 
 // Path helpers