[moetemp]

This PR lets a @nexus.helper method return at once, and lets it return a class of the support package.

• sync=true on @nexus.helper makes the Python method a plain def. It returns the helper's result with no await.
• @nexus.support-type python="<class>" on a type alias stands for a class that the support package exports. @nexus.generic on the alias gives its type parameters. nexgen generates nothing for the class, and only a helper method may return it.

The Workflow Stream gets a read that gives each Record as it arrives. Code iterates the result, and then reads from it where and why the read ended. So the method must hand back an object with no await, and that object has behavior, which a generated model can't have.

/// @nexus.support-type python="StreamReading"
/// @nexus.generic "body-t"
type stream-reading = placeholder;

/// @nexus.helper python="iter_stream" targets="workflow" sync=true
iter: func(timeout: option<duration>, from-offset: option<list<u8>>) -> stream-reading;

def iter(
    self, timeout: datetime.timedelta | None = None, from_offset: bytes | None = None
) -> _support.StreamReading[BodyT]:
    return _support.iter_stream(self, timeout=timeout, from_offset=from_offset)

The stream sample has the new method on its Workflow Stream. TypeScript, .NET, Go and Java leave it out, as they do for every helper method, so their sample outputs don't change.