fix: correct misplaced parentheses in _Spatialslip zeta corrections

[steps-re]

What

In the 3D branch of _Spatialslip (src/parcels/interpolators/_xinterpolators.py), the four vertical (zeta) slip corrections have a misplaced closing parenthesis:

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)),
    f_u * (a + b * zeta) / zeta,
    f_u,
)

The ) after is_land(0, 0, 1, 0 is missing, so 0 & is_land(0, 0, 1, 1) & (zeta > 0) is passed as is_land's 4th positional argument (the x-index) instead of 0. The & is_land(...) & (zeta > 0) mask that should gate np.where is swallowed into that argument, so both the land test and the zeta position gate are wrong. This breaks the vertical free-/partial-slip boundary correction for 3D C-grid velocities.

Fix

Move the parenthesis so each line matches the analogous, correctly-formed eta/xsi corrections in the same function:

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)

Same correction applied to all four affected lines (the zeta > 0 and zeta < 1 gates for f_u and f_v).

Testing

pytest tests/test_interpolation.py tests/test_convert.py passes (29 passed, 1 xfailed) with the change. Note the cgrid_velocity case in test_interpolation.py is currently commented out, which is likely why this went unnoticed.