Searches for $ \Lambda_b^0 $ and $ \Xi_b^0 $ decays to $ K_{\mathrm{S}}^0p{\pi^{-}} $ and $ K_{\mathrm{S}}^0p{K^{-}} $ final states with first observation of the $ \Lambda_b^0\to K_{\mathrm{S}}^0p{\pi^{-}} $ decay