[bug]: input payload on fetch paginated state

[yaythomas]

In an edge case where the Operations collection is empty on invoke due to size, it then later gets constructed via paginated api calls to

        execution_state.fetch_paginated_operations(
            invocation_input.initial_execution_state.operations,
            invocation_input.checkpoint_token,
            invocation_input.initial_execution_state.next_marker,
        )

https://github.com/aws/aws-durable-execution-sdk-python/blob/main/src/aws_durable_execution_sdk_python/execution.py#L330

The actual InputPayload will be on the EXECUTION operation after this fetch, whereas in the code currently it just uses the original input (which might be none)

        raw_input_payload: str | None = (
            invocation_input.initial_execution_state.get_input_payload()
        )

        input_event: MutableMapping[str, Any] = {}
        if raw_input_payload and raw_input_payload.strip():
            try:
                input_event = json.loads(raw_input_payload)
...
          user_future = executor.submit(func, input_event, durable_context)

aws-durable-execution-sdk-python/src/aws_durable_execution_sdk_python/execution.py

Line 330 in e5824f1

user_future = executor.submit(func, input_event, durable_context)

Note this is not actually happening today due to how backend currently works, nonetheless, this is a flaw in the SDK logic.