DynamoDB: transact_write_items does not return Item in CancellationReason (with example)

[cameron-toy]

#7523

Original issue was closed due to no reproducible test case

import pytest
import boto3
from moto import mock_aws
from botocore.exceptions import ClientError

@mock_aws
def test_transact_write_condition_check_failure():
    dynamodb = boto3.resource("dynamodb")
    client = boto3.client("dynamodb")
    
    table_name = "test-table"

    table = dynamodb.create_table(
        TableName=table_name,
        KeySchema=[{"AttributeName": "pk", "KeyType": "HASH"}],
        AttributeDefinitions=[{"AttributeName": "pk", "AttributeType": "S"}],
        BillingMode="PAY_PER_REQUEST"
    )
   
    table.put_item(Item={"pk": "key1", "foo": "bar"})

    try:
        client.transact_write_items(
            TransactItems=[
                {
                    "ConditionCheck": {
                        "TableName": table_name,
                        "Key": {"pk": {"S": "key1"}},
                        "ConditionExpression": "foo = :expected",
                        "ExpressionAttributeValues": {":expected": {"S": "nonexistent"}},
                        "ReturnValuesOnConditionCheckFailure": "ALL_OLD"
                    }
                }
            ]
        )
        pytest.fail("Expected TransactionCanceledException was not raised")
    except ClientError as e:
        error_code = e.response["Error"]["Code"]
        assert error_code == "TransactionCanceledException", f"Unexpected error code: {error_code}"
        
        cancellation_reasons = e.response.get("CancellationReasons", [])
        assert any(
            reason.get("Item") is not None for reason in cancellation_reasons
        ), "No cancellation reasons contain the 'Item' key"