Store DRep inline image data URI verbatim

[ArturWieczorek]

What

test_register_and_retire_drep (the drep_metadata_ipfs.json variant, which uses an
inline data:image/png;base64,… image) asserts the DRep off-chain image the old way.

cardano-db-sync #2138
(fixes #1966 stops stripping the data:<mime>;base64, prefix: per CIP-119 it now stores
the full contentUrl verbatim in off_chain_vote_drep_data.image_url. The check in
dbsync_utils.check_off_chain_drep_registration still expected the stripped base64
payload, so it fails against db-sync with #2138.

This updates the base64 branch to assert the full data URI (image_url == content_url).
Regular URL images (which carry a sha256) are unchanged.

⚠️ Merge coordination

The released 13.7.2.1 binary does not contain #2138, and db-sync master still reports
version 13.7.2.1, so the behavior can't be version-gated. After this merges,
test_register_and_retire_drep will fail against any db-sync without #2138 - including
CI-nightly, which pins released 13.7.2.1. Merge alongside bumping the nightly
DBSYNC_REV to the release that includes #2138.

Testing

To verify once a #2138 build exists:
pytest -k "test_register_and_retire_drep" cardano_node_tests/tests/tests_conway/test_drep.py