[FEA] COO interface still exposes some int nnz / int n_rows signatures (blocks >2B nonzeros)

[Intron7]

PR #2793 already templatized many COO kernels on nnz_t, which allows 64-bit nonzero counts. However, some parts of the COO interface (detail/coo.cuh and related helpers) still expose 32-bit int parameters or members for nnz/dimensions. This causes downstream bindings (e.g. cuML GraphHolder) to choke with OverflowError when trying to allocate >2.1B edges.