Fix dataset build runtime failures

[anth-volk]

The Modal dataset build has hit two runtime failures that should be fixed in the H5 4b branch:

• Public Census/ACS/CDC data downloads can fail transiently with 5xx responses or request timeouts. These should use capped exponential retry/backoff rather than treating the first transient failure as missing source data.
• The small enhanced CPS sparse HDF5 writer assumes string-like values expose decode_to_str(), but the runtime path can produce plain NumPy arrays. HDF5 serialization should handle both PolicyEngine encoded arrays and plain NumPy arrays.

This issue tracks adding the runtime fixes, targeted unit coverage, lint/format cleanup, and changelog coverage for the branch.