After a config change (e.g. switching model provider), the /new command
must clear the per-session _last_resolved_model cache so the next turn
resolves the model from the updated config instead of falling back to
the stale cached value.
Without this fix, if a transient config-cache miss occurs on the first
post-/new turn, the #35314 recovery path serves the old model from the
cache — the user sees the old model being used even though they changed
config.yaml and explicitly ran /new.
Fix applies to both call sites that reset session model state:
- GatewaySlashCommandsMixin._handle_reset_command (slash_commands.py)
- GatewayRunner compression-exhausted auto-reset (run.py)
Fixes#58403
