Pipeline behavior ordering previously depended on discovery/registration
order. Add an AddBehavior<T>(order:) API that captures the order at
registration time; the BehaviorTypeIndex now orders matching behaviors by
that explicit order (lower runs first), falling back to registration order
for ties. Removed the runtime Order property approach.
Multiple handlers for the same request were previously resolved via .First(),
with ordering depending on Type.GetInterfaces() order (non-deterministic).
Now ScalarRequestInvoker/StreamRequestInvoker throw InvalidOperationException
listing the candidate handlers when more than one matches.
