Add off-chain voting anchor tests

[ArturWieczorek]

Summary

Conway tests verifying cardano-db-sync's off-chain voting anchor handling (db-sync PR #2005, issue #1995). When an anchor's hash matches, db-sync stores it regardless of whether it parses, recording the outcome in off_chain_vote_data.is_valid:

• TRUE - valid JSON db-sync can decode (CIP-100 / CIP-108 / CIP-119)
• FALSE - valid JSON it cannot decode against any CIP schema (warning records why)
• NULL - not valid JSON (error object in json, raw bytes preserved)

Covered for both gov-action (info action) and DRep anchors.

Helper changes

• get_action_data / check_action_data: optional voting_anchor_id to select the correct row when several anchors share the same content hash; check_action_data now asserts is_valid is TRUE.
• GovActionProposalDBRow.param_proposal made nullable - it is NULL for info actions and only set for parameter-change actions. test_pparam_update still passes.

Anchor data files and URLs

Each vector lives in a committed file under cardano_node_tests/tests/data/, fetched by db-sync over HTTP from its public raw.githubusercontent.com URL. The URLs are kept as PUBLIC_ACTION_ANCHOR_* constants in common.py, following #3531 (no tinyurl). File names are short so the raw URL stays under the 128-byte on-chain anchor limit:

• ga_anchor.json - conformant (already on master)
• ga_anchor_nonconf.json - valid JSON, not CIP-decodable
• ga_anchor_invalid.json - not valid JSON

The last two were added with long names in #3497 and are renamed here.

Verification

Verified end-to-end on a local dev cluster - 5 passed on db-sync 13.7.2.1 (rev db8cdf6, which includes PR #2005):

5 passed in 1573.88s (0:26:13)

Note on CI / merge order

The ga_anchor_nonconf.json / ga_anchor_invalid.json URLs point at master, so the four non-conformant / invalid tests only go green once those renamed files are on master (the same bootstrap this PR originally had with #3497). The conformant test passes already. Local verification used a byte-identical shortlink for the two pending files - hashes are computed from the committed files, so db-sync behaves identically once the URLs resolve.

[ArturWieczorek]

Local cluster tests: 💚

============================= test session starts ==============================
platform linux -- Python 3.13.9, pytest-9.0.3, pluggy-1.5.0
metadata: cardano-node: 11.0.1, cardano-cli: 11.0.0.0, TESTNET_VARIANT: local_fast,
  CLUSTERS_COUNT: 1, HAS_CC: True, HAS_DBSYNC: True, db-sync: 13.7.2.1
  (rev db8cdf668cda632e9a09c30e1159fcb9fb34e952), HAS_SMASH: True, network magic: 42
rootdir: /home/artur/Projects/cardano-node-tests
configfile: pyproject.toml
collected 5 items

test_offchain_voting_anchor.py::TestGovActionAnchor::test_valid_voting_anchor_json@long PASSED [ 20%]
test_offchain_voting_anchor.py::TestGovActionAnchor::test_valid_voting_anchor_json_not_conforming_to_cip_100@long PASSED [ 40%]
test_offchain_voting_anchor.py::TestGovActionAnchor::test_invalid_voting_anchor_json@long PASSED [ 60%]
test_offchain_voting_anchor.py::TestDrepAnchor::test_drep_anchor_json_not_conforming@long PASSED [ 80%]
test_offchain_voting_anchor.py::TestDrepAnchor::test_drep_invalid_anchor_json@long PASSED [100%]

======================== 5 passed in 1573.88s (0:26:13) ========================

Full log:
cardano-node-tests-pr-3498-logs.txt