fix(copilot): refresh Redis session meta TTL during long-running turns

Root cause of empty session on reload: the session meta key in Redis has
a 1h TTL set once at create_session time. Turns exceeding 1h (like
session d2f7cba3 at 82min) cause the meta key to expire, making
get_active_session return False. The resume endpoint then returns 204
and the frontend shows an empty session.

Fix: publish_chunk now periodically refreshes the meta key TTL (every
60s) when session_id is provided. stream_and_publish already has
session_id and passes it through. This keeps the meta key alive for
as long as chunks are being published.

GCP logs confirmed the bug: at 09:49 (73 min into the turn),
GET_SESSION returned active_session=False, msg_count=1 — the meta
key had expired 13 minutes earlier.