Follow-up on the salvaged MoA restore fix:
- Extract the finally-block restore into _restore_moa_one_shot() so the
behavior is unit-testable without re-implementing it, and so the gateway
/moa handler and the finally block share one implementation.
- Restore the load-bearing #28686 zombie-eviction comment above
_release_running_agent_state that the original diff dropped.
- Rewrite the tests to call the real _restore_moa_one_shot helper (the
originals re-implemented the restore logic inline, so they passed
regardless of the production code).
The MoA one-shot restore ran inside the try block after
_handle_message_with_agent returned. When that call raised an
exception (agent init failure, interpreter shutdown, OOM), the
restore was skipped and the MoA model override stayed permanently
on _session_model_overrides — silently routing all subsequent
messages through the MoA reference fan-out with no user-visible
indication.
Move the restore to the finally block so it fires on every exit
path (success, exception, interrupt). The restore data lives on
the per-turn event object and would be lost if not consumed here.
