GH-50906: [C++] Support 1D tensors in SparseCSR/CSC matrix conversion

[pratyushadk]

Rationale for this change

Calling SparseCSRMatrix::Make or SparseCSCMatrix::Make on a 1D dense tensor currently returns a NotImplemented error ("TODO for ndim <= 1"). The CSR/CSC representation for a 1D vector is well-defined though: it is equivalent to a single-row sparse matrix where indptr = [0, nnz], indices holds the positions of non-zero values, and values holds the corresponding values. The COO format already handles 1D tensors, so this fills a straightforward gap.

What changes are included in this PR?

cpp/src/arrow/tensor/csx_converter.cc

The Convert() method in SparseCSXMatrixConverter now handles three cases cleanly. ndim == 0 returns Status::Invalid since scalars have no meaningful sparse representation. ndim == 1 takes a new path that does a single linear scan of the vector, writes indptr = [0, nnz], and records the positions and values of non-zeros. ndim == 2 is the existing path and is completely unchanged.

The reverse path (MakeTensorFromSparseCSXMatrix) had a hardcoded shape[1] access which would be out of bounds for a 1D tensor. That is fixed to use shape.size() > 1 ? shape[1] : shape[0].

cpp/src/arrow/sparse_tensor_test.cc

Three new tests added to the existing TestSparseCSRMatrix fixture covering the 1D case.

Are these changes tested?

Yes. Three new tests cover the main conversion path, the all-zero edge case, and a full round-trip from dense to sparse and back. All 153 tests in the sparse tensor test suite passed locally with no regressions.

Are there any user-facing changes?

SparseCSRMatrix::Make and SparseCSCMatrix::Make now accept 1D tensors instead of returning NotImplemented. There are no breaking changes.

• GitHub Issue: [C++] Support 1D tensors in SparseCSR/CSC matrix conversion #50906

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⚠️ GitHub issue #50906 has been automatically assigned in GitHub to PR creator.