hugr-py: `Hugr.add_order_link` breaks if source node has unconnected ports

[mark-koch]

from hugr import tys
from hugr.build import Dfg
from tket2_exts import quantum

QUANTUM_EXT = quantum()

dfg = Dfg(tys.Qubit)
meas = dfg.add_op(QUANTUM_EXT.get_op("MeasureFree").instantiate(), *dfg.inputs())
alloc = dfg.add_op(QUANTUM_EXT.get_op("QAlloc").instantiate())
dfg.hugr.add_order_link(meas, alloc)
dfg.set_outputs(alloc)

Validation fails with

Connected ports Port(Outgoing, 0) in Node(7) and Port(Incoming, 0) in Node(8) have incompatible kinds. Cannot connect bool to StateOrder.

The state order source is mistakenly attached to the bool output of the MeasureFree op.

This causes a bug in Guppy: #1092

[aborgna-q]

#2436 should fix this, could you try re-running the test on that branch?