Fix simSpawnObject argument count in the C++ client

[megavedl]

Fix simSpawnObject argument count in the C++ client

The C++ client sends five arguments, the server binds six, so every
simSpawnObject call from C++ fails on arity. It comes back as a generic
RPC error that says nothing about the cause, which makes it a slow one
to track down.

RpcLibServerBase.cpp:451   ..., scale, physics_enabled, is_blueprint
RpcLibClientBase.cpp:633   ..., scale, physics_enabled

The Python client already passes all six (PythonClient/cosysairsim/client.py:779),
so this is just the C++ side being out of step. Looks like it goes back
to 19a7009, which added is_blueprint to the server and the Python
client but not here.

Passing false, matching the Python default. Left the C++ signature
alone to keep this to the bug; exposing is_blueprint there would be a
separate change.

Tested against Blocks on UE 5.8.2, Linux: before, every spawn from the
C++ client threw; after, actors spawn and keep the name they were asked
for. simDestroyObject removes them again.