fix: block distribution fields on default galaxy clusters

[nightcityblade]

Summary

• fix the default galaxy cluster blocked-field list so distribution and sharing_group_id are checked separately
• add regression coverage for both blocked fields
• keep the existing galaxy fixture valid by marking its distributed cluster as non-default

Fixes #1458

Testing

• .venv/bin/python -m pytest -q tests/test_galaxy_cluster.py tests/test_mispevent.py (37 passed, 2 skipped, 2 subtests passed)
• .venv/bin/pre-commit run --files pymisp/mispevent.py tests/test_galaxy_cluster.py tests/mispevent_testfiles/galaxy.json
• .venv/bin/mypy pymisp/mispevent.py tests/test_galaxy_cluster.py

[Rafiot]

Looks good, thanks!