Same race as the WhatsApp bridge (#127047): a container/supervisor stop
signals the whole process tree, so the Photon sidecar (its own session)
can die before the gateway's stop flow reaches disconnect(), while
_inbound_running is still True. _supervise_sidecar() then reported
SIDECAR_CRASHED and queued a reconnect during shutdown.
Consult the runner's _stop_requested_by_signal, which the signal handler
sets before any stop work runs. A sidecar exit while the gateway keeps
running is unchanged: still a retryable fatal error.
Co-authored-by: liuhao1024 <sunsky.lau@gmail.com>
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