Science/Cleanup: Verify handling of surface stresses for PBL scheme vs. diffusion solver

Split off from HB/diffusion solver review at https://github.com/ESCOMP/atmospheric_physics/pull/270#discussion_r2301686221

In CAM's `vertical_diffusion.F90`, there is a note that
```fortran
case ( 'HB', 'HBR' )

! Modification : We may need to use 'taux' instead of 'tautotx' here, for
! consistency with the previous HB scheme.
```

the surface stresses passed to HB, the PBL scheme, are `tautotx` and `tautoty`, defined as `tautot = cam_in%ws + tautms + taublj`;

however, this is inconsistently handled compared to the diffusion solver (previously `compute_vdiff`), which use `taux` and `tauy`, defined as `taux = cam_in%wsx` (HB, UW) or `taux = 0` (CLUBB_SGS)

maybe the handling here could be refactored in the future. For now, to separate these inconsistent definitions of surface stresses, the suffixes `_for_holtslag_boville_boundary_layer_scheme` and `_for_vertical_diffusion` have been introduced to make this artificial distinction.