PipeMessage message deserialization has until now worked by calling JsonSerializer.Deserialize<T>(string) directly, which can return null in the case that the incoming string is the json literal "null". This has two unfortunate consequences:

1. Callers need to insist that the result for pipe messages in particular is not null by marking the call site with !, or else the nullability could spread from there.
2. In order to do so confidently, all calls to JsonSerializer.Deserialize<T>(string) need to be reviewed, whether relevant to these pipe messages or not, in order to instill the confidence to declare ! accurately. In fact, all of the pipe messages deserialized come from sources that would not be null during serialization, but it's been a nontrivial analysis each time.

This PR formalizes the path to serialize and deserialize PipeMessage types, giving the opportunity to check for null and fail fast, allowing this Deserialize method to be declared as returning a non-null message, letting call sites omit the risky ! assertion.