[BUG] dpa4spin accepts non-metal units but uses metal-unit hbar

### Summary

The `dpa4spin` and `dpa4spin/kk` pair styles accept non-metal physical unit styles, but their magnetic-force conversion always divides by a hard-coded reduced Planck constant in metal units (`eV·ps`). Magnetic dynamics are therefore unit-incorrect for accepted styles such as `real` and `si`.

This is present on `origin/master` at `8cfd46e37448`.

### Evidence

The constructor rejects only `units lj` and explicitly tells users that `real` is acceptable:

```cpp
if (strcmp(update->unit_style, "lj") == 0) {
// ... use a physical unit style such as metal or real.
}
```

Both implementations then use:

```cpp
constexpr double kHBar = 6.5821191e-04; // eV.ps
```

and scale the magnetic force with `force_unit_cvt_factor / kHBar`:

- `source/lmp/pair_dpa4spin.cpp:113,543-544`
- `source/lmp/pair_dpa4spin_kokkos.cpp:24,336`

For example, LAMMPS `real` units use `force->hplanck = 95.306976368` in kcal/mol·fs, so the numerator is converted while the denominator remains in eV·ps. The result is not the metal-unit result expressed in `real` units.

The repository's spin test helpers already state that only `metal` is valid, and LAMMPS's `PairSpin::settings()` rejects every non-metal unit style.

### Expected behavior

The safest fix is to reject `strcmp(update->unit_style, "metal") != 0` in `dpa4spin`, which also covers `dpa4spin/kk` through its base constructor. Alternatively, all magnetic-force dimensions would need a complete active-unit conversion based on `force->hplanck / (2*pi)` and the appropriate energy conversion.

A regression test should verify that `units real` fails at pair-style initialization with a clear error.

---

Coding agent: Codex
Codex version: codex-cli 0.149.0
Model: gpt-5.6-sol
Reasoning effort: xhigh