Preserve non-spatial coords through reproject() and merge()

[brendancol]

Closes #1454.

Summary

• reproject() and merge() rebuilt the output DataArray with only the y/x (and band) coords. Scalar coords like a single time timestamp, non-dimension coords like rioxarray's spatial_ref, and unrelated extra-dim coords were silently dropped.
• Both functions now copy any coord whose dims do not include the rebuilt spatial dims. Coords aligned to ydim or xdim are still dropped because their values are stale after the grid is rebuilt.
• Documents in the Returns section of both docstrings that the output y coordinate is always emitted in descending order (top-down, north-up) regardless of the input direction. This matches the standard raster convention.

Test plan

• pytest xrspatial/tests/test_reproject.py -x (155 passed)
• New TestCoordsPreservation class covers:
  • scalar time coord round-trip through reproject()
  • non-dim string coord round-trip
  • stale y-aligned coord is dropped
  • band coord round-trip on 3D input
  • first-raster scalar coord round-trip through merge()
  • y-descending output for ascending input on numpy and dask backends

[brendancol]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved in c1f9dca. The conflict in xrspatial/reproject/__init__.py combined both the non-spatial coords preservation from this branch and the attrs preservation logic from main. All 165 tests pass.

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