feat(stepfunctions-tasks): add configurable timeout

[lvthillo]

Issue # (if applicable)

Closes #36823.

Reason for this change

The CallAwsServiceCrossRegion task creates an internal Lambda function to perform the cross-region AWS API call, but its timeout is hardcoded to 30 seconds. Long-running API calls (e.g. copying a large S3 object across regions, polling slow control-plane APIs) cannot complete within 30s and there is currently no way for users to override this.

Description of changes

• Added a new optional lambdaTimeout?: Duration property on CallAwsServiceCrossRegionOptions. When unset, the
  default remains Duration.seconds(30), so existing behavior is preserved.
• The value is wired through to the underlying CrossRegionAwsSdkSingletonFunction.
• Added synth-time validation that the value is between 1 and 900 seconds (Lambda's allowed range). This is a
  fail-fast check for inputs that would always be rejected by CloudFormation at deploy time, so no feature flag is
  required.
• Updated the aws-stepfunctions-tasks README with a new section explaining the default and how to override it,
  including a copyObject example.
• Updated the existing integ.call-aws-service-cross-region integration test to exercise the new prop and
  regenerated its snapshot via a real deploy.

Naming note

The prop is named lambdaTimeout, not timeout. TaskStateBaseProps (the base props the construct extends)
already defines timeout as the Step Functions task timeout. Adding a second timeout prop with different
semantics on the merged interface would be confusing, so lambdaTimeout disambiguates that this controls the
internal Lambda's execution timeout rather than the Step Functions task timeout.

Describe any new or updated permissions being added

/

Description of how you validated changes

• Added unit tests:
  • default Lambda timeout is 30 seconds — backwards-compatibility
  • with custom lambdaTimeout — verifies Duration.minutes(15) produces Timeout: 900
  • fails when lambdaTimeout is out of range (0 seconds) and (901 seconds) via test.each
• Updated integ.call-aws-service-cross-region.ts to set lambdaTimeout: Duration.seconds(60) on the CreateTable
  task and regenerated the snapshot.
• Ran a full deploy of the integration test in us-east-1 via integ-runner --update-on-failed — the state machine
  executed end-to-end successfully against real AWS.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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