fix(server_job): don't call shutdown in on_exit

The work down in opencode_server.shutdown is already done by the on_exit
handler in opencode_server.spawn. Calling it again via
state.opencode_Server_job is likely to shutdown a newly spawned
replacement server

Author: cameronr

lua/opencode/server_job.lua

@@ -164,7 +164,7 @@ function M.ensure_server()
       promise:reject(err)
     end,
     on_exit = function(exit_opts)
-      state.opencode_server_job:shutdown()
+      promise:reject('Server exited')
     end,
   })