Union types break service serialization

What happened?

When we upgraded our conjure-python dependency we ran into runtime pyspark serialization issues. We previously could serialize a service object but post-conjure-python upgrade this same service was no longer serializable.

We suspect https://github.com/palantir/conjure-python/pull/320 or https://github.com/palantir/conjure-python/pull/221 broke serde behavior for us.

The pyspark error was:

py4j.protocol.Py4JJavaError: An error occurred while calling z:org.apache.spark.api.python.PythonRDD.runJob.
: org.apache.spark.SparkException: Job aborted due to stage failure: Task 0 in stage 1.0 failed 1 times, most recent failure: Lost task 0.0 in stage 1.0 (TID 1, localhost, executor driver): org.apache.spark.api.python.PythonException: Traceback (most recent call last):
  File "/opt/palantir/services/.4229/var/tmp/asset-install/85af169544daf00da129a002813aba21/spark/python/lib/pyspark.zip/pyspark/worker.py", line 413, in main
    func, profiler, deserializer, serializer = read_command(pickleSer, infile)
  File "/opt/palantir/services/.4229/var/tmp/asset-install/85af169544daf00da129a002813aba21/spark/python/lib/pyspark.zip/pyspark/worker.py", line 68, in read_command
    command = serializer._read_with_length(file)
  File "/opt/palantir/services/.4229/var/tmp/asset-install/85af169544daf00da129a002813aba21/spark/python/lib/pyspark.zip/pyspark/serializers.py", line 173, in _read_with_length
    return self.loads(obj)
  File "/opt/palantir/services/.4229/var/tmp/asset-install/85af169544daf00da129a002813aba21/spark/python/lib/pyspark.zip/pyspark/serializers.py", line 697, in loads
    return pickle.loads(obj, encoding=encoding)
AttributeError: type object 'AlertFailureResponse' has no attribute '_service_exception'

This was thrown when passing our service through a map function. This occurred even with zero data passed along. It was only the service code that previously worked.

Other conjure definitions:

      AlertResponse:
        union:
          failureResponse: AlertFailureResponse
          successResponse: AlertSuccessResponse

      AlertFailureResponse:
        fields:
          serviceException: ServiceException
      AlertSuccessResponse:
        fields:
          uuid: uuid

Our __conjure_generator_version__ is 3.12.1.

We mitigated the issue by building our Conjure service in a mapPartitions function which is likely a better practice anyway.

What did you want to happen?

We are not entirely sure on why these new type definitions are not serializable. I believe the fields are renamed in a way that pyspark's serialization cannot find but that is conjecture at this point.

Start by reproducing the failure when the generated service is passed to a PySpark map function with zero data, then compare the union definitions and the changes in conjure-python PRs 320 and 221. Done means the generated AlertFailureResponse and AlertSuccessResponse types, including ServiceException, can be serialized in this scenario without the _service_exception error.