BUG: Use @ instead of * for Affine multiplication (#937)

[VolodymyrLinuxovich]

• Closes rioxarray.open_rasterio starting to emit warning from affine #937
• Tests added
• Fully documented, including docs/history.rst for all changes and docs/rioxarray.rst for new API

Problem

affine 3.0.1 (2026-08-28) emits PendingDeprecationWarning: Use @ matmul instead of * mul operator for matrix multiplication whenever * is used, both for composing transforms and for applying a transform to coordinates. open_rasterio triggers it through affine_to_coords, and rio.transform(recalc=True) triggers it through XRasterBase.transform.

Change

• rioxarray/_spatial_utils.py and rioxarray/rioxarray.py: replace the 5 Affine * multiplications with @. Running the test suite with that warning turned into an error finds no other call sites in rioxarray.
• pyproject.toml: add affine>=3.0. @ was added in affine 3.0, and rasterio 1.4.3 still accepts affine 2.x. rioxarray already imports affine directly in several modules, so this also declares a dependency it was relying on implicitly. If you'd rather keep supporting affine 2.x, I can switch to a version-agnostic approach.
• affine_to_coords wraps its return values in numpy.asarray (a no-op at runtime). This is needed because mypy picks a float return type from affine's __matmul__ overloads, which would otherwise add two new dict-item errors.

Some warnings remain from inside rasterio itself (rasterio.windows, rasterio.transform, rasterio.merge). Those need a fix in rasterio.

Tests

• New test_open_rasterio__no_affine_mul_warning, parametrized for rectilinear and rotated transforms so it covers both affine_to_coords branches. It opens the file from the issue with that warning turned into an error and calls rio.transform(recalc=True). It fails on master and passes with this PR.
• pre-commit passes. pylint rates the changed modules 10.00/10. mypy rioxarray/ shows the same 3 pre-existing operator errors in _spatial_utils.py as master, because affine's type hints don't cover NumPy arrays.
• Full suite, locally (pip wheels: rasterio 1.5.1 / GDAL 3.12.4, affine 3.0.1): no new failures. The same 91 tests fail on master in this environment because the wheel's GDAL has no netCDF/HDF drivers. Warnings drop from 2787 to 672.

[VolodymyrLinuxovich]

Closing as a duplicate of #939, which already does this with the requested pins.