fix(aws-apigateway): missing comma to make failure response payload valid json (#19253)

The failure response from a StepFunctionsRestApi integration is not valid JSON, it is missing a comma.  This fixes the issue by adding a comma.  Fixes #19252

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: rogerchi

packages/@aws-cdk/aws-apigateway/lib/integrations/stepfunctions.ts

@@ -203,7 +203,7 @@ function integrationResponse() {
           '#if($input.path(\'$.status\').toString().equals("FAILED"))', 
             '#set($context.responseOverride.status = 500)', 
             '{',
-              '"error": "$input.path(\'$.error\')"',
+              '"error": "$input.path(\'$.error\')",',
               '"cause": "$input.path(\'$.cause\')"',
             '}',
           '#else',

packages/@aws-cdk/aws-apigateway/test/integ.stepfunctions-api.expected.json

@@ -160,7 +160,7 @@
           "IntegrationResponses": [
             {
               "ResponseTemplates": {
-                "application/json": "#set($inputRoot = $input.path('$'))\n#if($input.path('$.status').toString().equals(\"FAILED\"))\n#set($context.responseOverride.status = 500)\n{\n\"error\": \"$input.path('$.error')\"\n\"cause\": \"$input.path('$.cause')\"\n}\n#else\n$input.path('$.output')\n#end"
+                "application/json": "#set($inputRoot = $input.path('$'))\n#if($input.path('$.status').toString().equals(\"FAILED\"))\n#set($context.responseOverride.status = 500)\n{\n\"error\": \"$input.path('$.error')\",\n\"cause\": \"$input.path('$.cause')\"\n}\n#else\n$input.path('$.output')\n#end"
               },
               "StatusCode": "200"
             },

packages/@aws-cdk/aws-apigateway/test/integrations/stepfunctions.test.ts

@@ -418,7 +418,7 @@ function getIntegrationResponse() {
           '#if($input.path(\'$.status\').toString().equals("FAILED"))',
           '#set($context.responseOverride.status = 500)',
           '{',
-          '"error": "$input.path(\'$.error\')"',
+          '"error": "$input.path(\'$.error\')",',
           '"cause": "$input.path(\'$.cause\')"',
           '}',
           '#else',

packages/@aws-cdk/aws-apigateway/test/stepfunctions-api.test.ts

@@ -181,7 +181,7 @@ function getIntegrationResponse() {
           '#if($input.path(\'$.status\').toString().equals("FAILED"))',
           '#set($context.responseOverride.status = 500)',
           '{',
-          '"error": "$input.path(\'$.error\')"',
+          '"error": "$input.path(\'$.error\')",',
           '"cause": "$input.path(\'$.cause\')"',
           '}',
           '#else',