OSHConnect.add_system_to_node / create_and_insert_system silently no-op on an unregistered node

[tipatterson-dev]

Problem

OSHConnect.add_system_to_node() and OSHConnect.create_and_insert_system() (oshconnectapi.py:297 and :310) both guard their whole body on if target_node in self._nodes: with no else. When the node isn't registered with the OSHConnect instance, they fall off the end and return None — no exception, no log line. Nothing is POSTed and nothing is attached, but the caller has no way to tell that from a success.

create_and_insert_system() is the worse of the two: its documented return is "the created system", so a caller doing sys = osh.create_and_insert_system(opts, node) gets None and dies later on None.add_insert_datastream(...) — the same class of delayed, misattributed failure as #42, just with a different trigger (the POST is never attempted at all rather than rejected).

Reproduce

osh = OSHConnect(name="demo")
node = Node(protocol="http", address="localhost", port=8282)  # never osh.add_node(node)

sys = osh.create_and_insert_system({"label": "S", "urn": "urn:x:1"}, node)
assert sys is None          # silently

Same for osh.add_system_to_node(system, unregistered_node) — returns None and the system is in neither osh.systems() nor node.systems().

Possible fix

Raise on the miss rather than falling through:

if target_node not in self._nodes:
    raise ValueError(
        f"Node {target_node.get_id()} is not registered with this "
        f"OSHConnect instance; call add_node() first."
    )

ValueError matches what add_datastream() already raises a few lines up for an unresolvable system id, so it's consistent with the surrounding module. Worth auditing the rest of oshconnectapi.py for the same if …: … return shape while in there.

Related: #42 (same silent-failure family, POST-rejected trigger).