resample: preserve scalar non-dim coords and refresh stale nodata attr (#2542)

[brendancol]

Summary

• The 2D non-identity path in resample() rebuilt its output xr.DataArray with only the spatial dim-coords, silently dropping any scalar non-dim coord on the input (e.g. rioxarray's spatial_ref, or a squeezed time / band selector). The identity path (scale_factor=1.0) and the 3D per-band path both kept these coords, so the 2D path was the inconsistent outlier (Cat 5).
• _resolve_nodata() reads attrs['nodata'] as a sentinel fallback, but the output post-processing only refreshed _FillValue and nodatavals. Inputs declaring attrs['nodata'] = -9999 came back with that same finite value alongside data that was now NaN (Cat 4).
• Fixed both gaps in one place at the final xr.DataArray constructor. Spatially-shaped non-dim coords intentionally do not carry across; their length is tied to the input resolution.

Backend coverage

numpy / cupy / dask+numpy / dask+cupy all route through the same constructor. Verified end-to-end for the spatial_ref carry across all four backends. Nodata-attr refresh verified on numpy / cupy / dask+numpy; dask+cupy with a non-NaN sentinel hits a pre-existing xr.where + cupy.astype quirk unrelated to this audit.

Test plan

• 175 tests pass in xrspatial/tests/test_resample.py (168 existing + 7 new)
• New TestMetadataPropagation cases: nodata-attr refresh, spatial_ref carry, scalar band/time carry, identity-vs-downsample coord parity, intentional drop of spatially-shaped extras
• 4-backend spatial_ref parity confirmed manually
• Discovered via deep-sweep metadata audit (sweep-metadata, 2026-05-27)

Closes #2542

[brendancol]

PR Review: preserve scalar non-dim coords and refresh stale nodata attr

Read source (resample.py:1300-1424) and tests in full from the worktree at the PR head commit. The fix lives at the single 2D output constructor, so all 4 backends inherit it without per-backend changes.

Blockers

None.

Suggestions

• xrspatial/tests/test_resample.py:203-308 — the PR body claims 4-backend parity for spatial_ref carry was verified manually, but no automated test pins this. Since the fix lives at the shared xr.DataArray constructor, a single @cuda_and_cupy_available test asserting spatial_ref survives a cupy resample would catch a backend-divergent regression. The companion PR (resample: close test-coverage gaps surfaced by deep-sweep audit #2548) already has the cross-backend scaffolding to copy from.
• xrspatial/resample.py:1404-1412 — the 3D path (lines 1290-1322) hands each band to the 2D resample() call, so this fix transitively propagates spatial_ref through xr.concat. Worth one explicit test confirming the 3D spatial_ref case so the contract is pinned, not just inferred.

Nits

• xrspatial/resample.py:1418 — coords={ydim: new_y, xdim: new_x, **extra_coords} relies on the coord_name in (ydim, xdim) skip at line 1409 to keep extra_coords from shadowing the rebuilt spatial coords. The current filter prevents collision; the safety is one-sided though (if the filter ever loosens, **extra_coords would silently override).
• xrspatial/tests/test_resample.py:228,247 — int(out.coords['spatial_ref'].values) == 0 and int(out.coords['band'].values) == 1 cast the scalar coord to int. Works for the values used; == 0 / == 1 against the np scalar is one less coercion to read.

What looks good

• Fix is at the single shared 2D output constructor, so all four backends pick it up without per-backend code.
• Comments on lines 1387-1393 and 1396-1402 explain the why: which input attrs feed _resolve_nodata, why the identity path was the consistent outlier, why spatially-shaped coords are intentionally dropped.
• Test names mirror the audit categories (test_nodata_attr_refreshed_to_nan, test_identity_and_downsample_coords_consistent), so a failure maps straight back to the issue.
• test_nodata_attr_refreshed_to_nan pins both the attr refresh AND that the corresponding data pixel is NaN — catches the exact "metadata says NaN but data still has -9999" failure mode that motivated the fix.
• test_2d_spatial_extra_coord_not_carried explicitly documents the chosen scope: scalar coords carry, spatially-shaped coords don't.

Checklist

• Behavior matches issue resample: drops non-dim coords (spatial_ref) and leaves stale attrs['nodata'] on 2D path #2542
• All implemented backends share the fix at the constructor seam
• NaN handling correct (nodata attr now matches data state)
• Edge cases covered (nodata absent stays absent, 2D spatial coord drops, identity vs downsample parity)
• No premature materialization
• Docstrings present and accurate (no signature change)