`sess` targets a dead connection after reload if session watcher turns from on to off

If session watcher turns from on to off, and VS Code is reloaded, the existing R process would retain the previously installed sess hooks. Those can target the dead connection, breaking viewers such as

> plot(1:10)
warning messages from top-level task callback 'sess.workspace'
Warning message:
In value[[3L]](cond) :
  [sess] Failed to send IPC message: Native call to `processx_connection_write_bytes` failed
> View(dat)
Warning message:
In value[[3L]](cond) :
  [sess] Failed to send IPC message: Native call to `processx_connection_write_bytes` failed
warning messages from top-level task callback 'sess.workspace'
Warning message:
In value[[3L]](cond) :
  [sess] Failed to send IPC message: Native call to `processx_connection_write_bytes` failed
> help(glm)
Warning message:
In value[[3L]](cond) :
  [sess] Failed to send IPC message: Native call to `processx_connection_write_bytes` failed
warning messages from top-level task callback 'sess.workspace'
Warning message:
In value[[3L]](cond) :
  [sess] Failed to send IPC message: Native call to `processx_connection_write_bytes` failed

Start by reproducing the session-watcher transition from on to off, reload VS Code with the existing R process still running, and observe the retained sess hooks. The fix is complete when viewers and commands such as plot(), View(), and help() no longer attempt to send IPC messages to the dead connection.