Reproduces the proxy failing to reconnect to a destination relay when the
sender disconnects mid-connection (empty session var left in smpClients).

getSessVar inserts an empty session var that the connect path then fills with
putTMVar. If the connecting thread is killed by an async exception before that
fill (a proxy worker on client disconnect, an agent worker on cancel), the empty
var was left in the map forever and every later request for that server blocked
on it until timing out (permanent PCEResponseTimeout).

Wrap get-or-create with withGetSessVar (bracketOnError) at the call sites, so the
cleanup is established where the var is created and covers the whole connect: on
interrupt before fill the still-empty var is dropped and the next request
reconnects. This closes the window between getSessVar and the fill that a handler
installed inside the connect function cannot cover.