fix: copy connectivities before passing to umap-learn

[nbjustice]

• Closes sc.tl.umap() silently mutates adata.obsp['connectivities'] via shared sparse buffer #4028
• Tests included
• Release notes included

Summary

sc.tl.umap() passes neighbors["connectivities"].tocoo() to umap-learn's simplicial_set_embedding() without copying. Since csr_matrix.tocoo() defaults to copy=False, the COO and CSR matrices share the same data buffer. umap-learn then zeroes out small edge weights in-place, which silently corrupts adata.obsp['connectivities'].

Fix: .tocoo() → .tocoo(copy=True)

Test plan

• Added test_umap_preserves_connectivities asserting data values, nnz count, no explicit zeros, and embedding is still produced
• Verified test fails without the fix and passes with it
• All existing UMAP tests pass