Client-facing builder for the per-run graph context payload.
Callers populate this and pass it via context= to astream/ainvoke.
ConfigurableModelMiddleware and the interrupt_on when predicate read
it from request.runtime.context. In-process LangGraph coerces it into
CLIContextSchema (the registered context_schema); over the API it stays
a plain dict — which is why consumers handle both shapes.