Minor follow-up to the native-image-buffer isolation fix. The write site
in _prepare_inbound_message_text was calling build_session_key directly,
while every other call site in gateway/run.py uses the _session_key_for_source
helper — which consults session_store._generate_session_key first and falls
back to build_session_key. Keeping the write key and consume key on the
same helper prevents key drift if the session store ever overrides the
default keying behavior.
a178081468