fix: add explicit nspin=4 to 816_PW_DFTU_S4_XY test INPUT

Commit b95f433 on develop removed the automatic nspin=4 reset
when noncolin/lspinorb is enabled, instead requiring explicit nspin=4.
Since the PR does not touch read_input_item_elec_stru.cpp, the CI merge
uses develop's validation code, causing ABACUS to quit with:
  nspin must be 4 when noncolin or lspinorb is enabled.

Author: dyzheng

tests/01_PW/816_PW_DFTU_S4_XY/INPUT

@@ -7,6 +7,7 @@ gamma_only    0
 device    cpu
 
 noncolin    1
+nspin    4
 scf_thr    1.0e-6
 scf_nmax    50
 out_chg    0