fix(apigateway): correct invalid JSON in StepFunctions 500 response template

[aemada-aws]

Issue # (if applicable)

Closes #34777.

Reason for this change

The 500 response template in StepFunctions integration contains invalid VTL (Velocity Template Language) that produces malformed JSON, causing API Gateway to throw transformation errors when processing 5XX responses.

Description of changes

Fixed the 500 response template by wrapping the error field in proper JSON braces:

• Changed "error": $input.path('$.error') to { "error": "$input.path('$.error')" }
• Updated corresponding test to expect the corrected JSON format

This ensures the response template generates valid JSON that API Gateway can properly transform without errors.

Describe any new or updated permissions being added

No new permissions required.

Description of how you validated changes

• Ran existing unit tests to ensure no regressions
• All StepFunctions integration tests pass with the corrected template format

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Fixes must contain a change to an integration test file and the resulting snapshot.

If you believe this pull request should receive an exemption, please comment and provide a justification. A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed, add Clarification Request to a comment.

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 07a7135
• Result: FAILED
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[aemada-aws]

Closed in favor of #34787

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