On a compute-host (turn_isolation) session the live agent lives in the child
process, but config.set model either stashed pending_model_switch for a turn
thread that pops-and-drops it against agent=None (busy), or built a second
agent in the serving process and switched that copy while the child kept the
old model (idle). Carry the stash in the turn frame (copied, not popped), let
the child's _ensure_server_session adopt it in both branches, and clear the
server-side stash only after a successful isolated turn.
Rebases tsemana's #79510 onto the post-split modules: the frame key and the
clear-on-success now live in compute_host_bridge.py, adoption in
compute_host.py, and the always-defer condition in methods_config_set.py.
Fixes#79509
Co-authored-by: Tony Semana <tsemana@users.noreply.github.com>
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