Update python runtime to supported version

Support for python3.9 runs out on October 30, 2025. This change ensures
that we remain in support and this example continues working after
February 3, 2026.

See https://docs.aws.amazon.com/lambda/latest/dg/lambda-runtimes.html#runtime-support-policy

Author: DavidS-ovm

modules/scenarios/message-size-breach/main.tf

@@ -46,7 +46,7 @@ resource "aws_lambda_function" "image_processor" {
   role            = aws_iam_role.lambda_role.arn
   handler         = "lambda_function.lambda_handler"
   source_code_hash = filebase64sha256("${path.module}/lambda_function.zip")
-  runtime         = "python3.9"
+  runtime          = "python3.13"
   timeout         = var.lambda_timeout
 
   # This will fail when batch size × message size > 256KB (Lambda async limit)