Refactor connectivity computation into helper function

[valleyofdawn]

Currently, FastTag relies on a connectivity matrix that is usually generated during the preprocessing pipeline.

To decouple FastTag from specific preprocessing workflows (e.g., scanpy pipelines), we should:

• Move connectivity calculation (e.g., sc.pp.neighbors) to an optional helper function.
• Have FastTag raise an error if .obsp['connectivities'] is missing.
• Optionally provide a utility like prepare_adata_for_fasttag() for beginners.

This will improve modularity, portability, and reduce unnecessary dependencies.