The gateway /model command stored session overrides in
_session_model_overrides but run_sync() never consulted them when
resolving the model and runtime for the next message. It always read
from config.yaml, so the switch was lost as soon as a new agent was
created.
Two fixes:
1. In run_sync(), apply _session_model_overrides after resolving from
config.yaml/env — the override takes precedence for model, provider,
api_key, base_url, and api_mode.
2. In post-run fallback detection, check whether the model mismatch
(agent.model != config_model) is due to an intentional /model switch
before evicting the cached agent. Without this, the first message
after /model would work (cached agent reused) but the fallback
detector would evict it, causing the next message to revert.
Affects all gateway platforms (Telegram, Discord, Slack, WhatsApp,
Signal, Matrix, BlueBubbles, HomeAssistant) since they all share
GatewayRunner._run_agent().
Fixes#6213
