fix(smithy-aws-core): omit Result wrapper for empty awsQuery output

[Alan4506]

Problem:

The awsQuery protocol returns XML responses wrapped in two layers: an outer <{OperationName}Response> element, and inside it an <{OperationName}Result> element and a <ResponseMetadata> element. For example, Amazon SNS CreateTopic returns:

<CreateTopicResponse xmlns="http://sns.amazonaws.com/doc/2010-03-31/">
  <CreateTopicResult>
    <TopicArn>arn:aws:sns:us-east-1:111122223333:my-topic</TopicArn>
  </CreateTopicResult>
  <ResponseMetadata>
    <RequestId>...</RequestId>
  </ResponseMetadata>
</CreateTopicResponse>

To deserialize this, AwsQueryClientProtocol._response_wrapper_elements tells the XML deserializer to descend through both wrapper elements before reading members. The current implementation always returns both:

def _response_wrapper_elements(
    self,
    operation: APIOperation[SerializeableShape, DeserializeableShape],
) -> tuple[str, str]:
    return (
        f"{operation.schema.id.name}Response",
        f"{operation.schema.id.name}Result",
    )

The problem is that some operations' output shapes do not have members, so they do not get a <{OperationName}Result> element from the service. The response body contains only <ResponseMetadata> inside the outer wrapper. Requiring <{OperationName}Result> then fails.

This was found while testing Amazon SNS SetTopicAttributes and DeleteTopic operations. For example:

<SetTopicAttributesResponse xmlns="http://sns.amazonaws.com/doc/2010-03-31/">
  <ResponseMetadata>
    <RequestId>...</RequestId>
  </ResponseMetadata>
</SetTopicAttributesResponse>

The call failed with:

XMLParseError: Error parsing XML: Expected element 'SetTopicAttributesResult', got 'ResponseMetadata'

Description of changes:

In _response_wrapper_elements, only include the <{OperationName}Result> wrapper when the operation's output shape has members. Operations with empty output are unwrapped using just the outer <{OperationName}Response>. The return type is changed from tuple[str, str] to tuple[str, ...] since it now returns either one or two wrapper names.

Testing:

• Added a unit test in tests/unit/aio/test_protocols.py that deserializes an empty-output awsQuery response, and asserts it returns the output shape instead of raising.
• Generated Amazon SNS client locally, and verified its operation worked well: CreateTopic / GetTopicAttributes / SetTopicAttributes / ListTopics / DeleteTopic / Subscribe.

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