Add pre-computed neighbors to PCA h5ad files

[nictru]

Summary

• Adds pre-computed neighbor graphs (sc.pp.neighbors(adata, use_rep='X_pca'), default parameters) to pbmc/SRR28679759_filtered_pca.h5ad and pbmc/combined_filtered_pca.h5ad
• These files already contained pre-computed PCA embeddings; this extends them with the neighbor graph (obsp/connectivities, obsp/distances, uns/neighbors) so downstream module tests no longer need to run SCANPY_NEIGHBORS in their setup blocks

Context

These files are used by the scdownstream pipeline module tests. Adding pre-computed neighbors allows removing the SCANPY_NEIGHBORS step from test setup blocks, reducing test complexity and eliminating a source of numerical non-determinism across platforms.