Restore = nullptr defaults to CUDA Rdr2Geo.topo() optional rasters (#265)

[s-sasaki-earthsea-wizard]

Closes #265.

Summary

The CUDA pybind binding for isce3::cuda::geometry::Topo::topo()
declared all 11 output-raster kwargs as required, while the underlying
C++ class accepts nullptr per its documented contract and the CPU
sibling binding marks the same kwargs = nullptr. Calling the CUDA
overload with a subset of layers (kwargs only, others omitted) raised
TypeError on argument matching:

import isce3.cuda.geometry as g
rdr2geo = g.Rdr2Geo(radar_grid, orbit, ellipsoid, doppler)
rdr2geo.topo(dem_raster, x_raster=x, y_raster=y, height_raster=z)
# TypeError: topo(): incompatible function arguments

The CPU sibling accepts the same call and produces the requested
layers. Full evidence — including pybind11 docstring inspection and a
self-contained repro that imports only isce3 — is in #265.

What's in this PR

Two commits:

1. Restore = nullptr defaults to CUDA Rdr2Geo.topo() optional rasters
   — adds = nullptr to the 11 output-raster kwargs in
   python/extensions/pybind_isce3/cuda/geometry/rdr2geo.cpp to mirror
   the CPU sibling and the C++ class signature
   (cxx/isce3/cuda/geometry/Topo.h:43-54). Also renames
   local_Psi_raster → local_psi_raster for parity with the CPU
   binding (the lowercase form was already used by the CPU sibling and
   by the shared docstring; the capital-P was a leftover from the same
   rewrite that dropped the defaults).
2. Add CUDA Rdr2Geo.topo() optional-rasters regression test —
   test_run_optional_layers in
   tests/python/extensions/pybind/cuda/geometry/rdr2geo.py exercises
   the kwarg-subset call shape (subset of layers requested, others
   passed as None or omitted) so future binding-layer regressions on
   the optional path get caught. Also gates the whole module with
   pytestmark = pytest.mark.skipif on isce3.cuda availability so
   CPU-only builds skip cleanly with a reason rather than failing
   fixture collection.

Net diff: ~15 lines of binding code (11 = nullptr + the rename) plus a
regression test. No kernel changes, no C++ class changes — purely a
binding-layer restoration.

Why this is regression-shaped, not a feature

The defaults existed historically. The "Regression history" section in
#265 traces this: commit a19944c0 (2022, "Optional topo layers") added
= nullptr to both the CPU and CUDA bindings symmetrically. The
later commit 362813e7 (2023, "ground-to-sat East/North components")
rewrote the CUDA binding block in a different formatting style and
silently dropped every = nullptr along the way, while the CPU sibling
was edited in place and kept its defaults. This PR restores the
pre-362813e7 contract for the CUDA side.

Why existing CI didn't catch it

tests/python/extensions/pybind/cuda/geometry/rdr2geo.py::test_run_raster_layers
exercises the multi-raster overload but always passes every raster
kwarg explicitly. No test ever exercised the optional path on the CUDA
side, so CI was green through 362813e7. The new
test_run_optional_layers covers the gap.

Internal NISAR callers (nisar/workflows/{rdr2geo,geocode_corrections}.py)
pass all 12 raster args positionally with the unwanted ones as None.
pybind11 converts None → nullptr for Raster* arguments regardless of
whether the binding declares a default, so those callers are insensitive
to the missing defaults. Only kwarg-subset callers hit it.

Tests

pytest tests/python/extensions/pybind/cuda/geometry/rdr2geo.py -v —
5/5 PASS (existing 4 + the new test_run_optional_layers) on a
CUDA-enabled build. On a CPU-only build the whole module reports a
clean skip via pytestmark.

The standalone isce3-only repro in #265 ("Reproducing" section) goes
from CPU defaults=11/PASS, CUDA defaults=0/TypeError (BEFORE) to
CPU defaults=11/PASS, CUDA defaults=11/PASS (AFTER) on this branch.

Notes

• #868 and #1393 referenced by a19944c0 and 362813e7 commit
  messages are JPL-internal MCR numbers (GitLab), not GitHub PRs.
• The C++ class signature is unchanged; this is purely binding-layer.
• Rebased onto current develop HEAD 2919e1c97 (no conflicts —
  upstream files touched by intervening commits Expose sensing start/end times and starting/ending ranges in the STATIC runconfig #205, Fix recently-introduced mixed-mode bug #268, Scale RSLC mixed-mode filter by bandwidth ratio #269 are
  disjoint from the two files in this PR).