fix: correct misplaced parentheses in _Spatialslip zeta corrections (#2743)

In the 3D branch of _Spatialslip, the four vertical (zeta) slip corrections
had a misplaced closing parenthesis, e.g.:

    is_land(0, 0, 1, 0 & is_land(0, 0, 1, 1) & (zeta > 0))

so the `& is_land(...) & (zeta > 0)` mask meant to gate np.where was instead
swallowed into is_land's 4th positional argument (the x-index). Both the land
test and the zeta position gate were therefore wrong, breaking the vertical
free-/partial-slip boundary correction for 3D C-grid velocities.

Fixed to match the analogous, correctly-formed eta/xsi corrections in the same
function:

    is_land(0, 0, 1, 0) & is_land(0, 0, 1, 1) & (zeta > 0)

Existing interpolation and convert tests pass.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Erik van Sebille <e.vansebille@uu.nl>

Author: 3 people

src/parcels/interpolators/_xinterpolators.py

@@ -455,22 +455,22 @@ def is_land(ti: int, zi: int, yi: int, xi: int):
             f_v,
         )
         f_u = np.where(
-            is_land(0, 0, 0, 0) & is_land(0, 0, 0, 1) & is_land(0, 0, 1, 0 & is_land(0, 0, 1, 1) & (zeta > 0)),
+            is_land(0, 0, 0, 0) & is_land(0, 0, 0, 1) & is_land(0, 0, 1, 0) & is_land(0, 0, 1, 1) & (zeta > 0),
             f_u * (a + b * zeta) / zeta,
             f_u,
         )
         f_u = np.where(
-            is_land(0, 1, 0, 0) & is_land(0, 1, 0, 1) & is_land(0, 1, 1, 0 & is_land(0, 1, 1, 1) & (zeta < 1)),
+            is_land(0, 1, 0, 0) & is_land(0, 1, 0, 1) & is_land(0, 1, 1, 0) & is_land(0, 1, 1, 1) & (zeta < 1),
             f_u * (1 - b * zeta) / (1 - zeta),
             f_u,
         )
         f_v = np.where(
-            is_land(0, 0, 0, 0) & is_land(0, 0, 0, 1) & is_land(0, 0, 1, 0 & is_land(0, 0, 1, 1) & (zeta > 0)),
+            is_land(0, 0, 0, 0) & is_land(0, 0, 0, 1) & is_land(0, 0, 1, 0) & is_land(0, 0, 1, 1) & (zeta > 0),
             f_v * (a + b * zeta) / zeta,
             f_v,
         )
         f_v = np.where(
-            is_land(0, 1, 0, 0) & is_land(0, 1, 0, 1) & is_land(0, 1, 1, 0 & is_land(0, 1, 1, 1) & (zeta < 1)),
+            is_land(0, 1, 0, 0) & is_land(0, 1, 0, 1) & is_land(0, 1, 1, 0) & is_land(0, 1, 1, 1) & (zeta < 1),
             f_v * (1 - b * zeta) / (1 - zeta),
             f_v,
         )