[Testing] Remove references of `replace` in favour of creating immutable objects

[dhegberg]

Migration from aws/aws-durable-execution-sdk-python-testing#115

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Let's remove anywhere we're using replace() in favour of instantiating a new instance of a given class. We want to enforce a consistent style of using immutable objects.

See https://github.com/aws/aws-durable-execution-sdk-python-testing/blob/main/src/aws_durable_execution_sdk_python_testing/model.py#L2448-L2461 for example.

Brought up in this PR: aws/aws-durable-execution-sdk-python-testing#95 (comment)