chore: remove snapshot creation/restoration

[OlufemiAdeOlusile]

Summary

We do not run db-sync snapshot workflows anymore, so this removes
the whole subsystem rather than leaving unused code and CI toggles
sitting around.

Removed:

• test_snapshot_creation.py, test_local_snapshot_restoration.py,
  test_iohk_snapshot_restoration.py
• sync_tests/utils/db_sync/snapshots.py and its re-export
  wrappers in db_sync/init.py
• teardown_node_and_db_sync() in sync_entries.py, which existed
  only to serve the two restoration test fixtures
• the snapshot_creation, local_snapshot, and iohk_snapshot
  pytest markers
• the --snapshot-url and --run-only-sync-test CLI options
• the run_only_sync_test toggle in db_sync_full_sync.yaml, which
  only existed to gate the now-removed snapshot tests

Not touched: peer-snapshot handling in sync_tests/utils/node
(_downgrade_peer_snapshot, normalize_peer_snapshot, etc). That is
unrelated node topology bootstrap data, not db-sync snapshotting.

Note for reviewers: PR #162 (unit tests workflow) filters pytest
with -m "not (... or snapshot_creation or local_snapshot or iohk_snapshot ...)". Those marker names will just no longer match
anything once this merges; nothing breaks, but that filter list
should get cleaned up in a follow-up on #162.

Test plan

• ruff check sync_tests/ .github/ passes
• ruff format --check passes
• mypy sync_tests/ passes with no issues
• pytest sync_tests/tests/ (non-sync markers) - 15 passed
• grep confirms no remaining references to removed functions,
  files, markers, or CLI options

[Copilot]

Pull request overview

Removes the obsolete Cardano db-sync snapshot creation/restoration subsystem from the test suite, CLI, and CI workflow configuration now that snapshot workflows are no longer run.

Changes:

• Deletes snapshot creation/restoration tests and the underlying db_sync.snapshots utility module (plus its re-export wrappers).
• Removes snapshot-related pytest markers and CLI options from test configuration.
• Simplifies the db-sync full-sync GitHub Actions workflow to always run only node_sync / db_sync tests.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
sync_tests/utils/sync_entries.py	Removes teardown helper that existed only for snapshot restoration fixtures.
sync_tests/utils/db_sync/snapshots.py	Deletes snapshot download/create/restore implementation.
sync_tests/utils/db_sync/init.py	Drops snapshots import and removes snapshot-related wrapper APIs.
sync_tests/tests/test_snapshot_creation.py	Deletes snapshot creation test coverage (no longer used).
sync_tests/tests/test_local_snapshot_restoration.py	Deletes local snapshot restoration tests (no longer used).
sync_tests/tests/test_iohk_snapshot_restoration.py	Deletes IOHK snapshot restoration tests (no longer used).
sync_tests/tests/conftest.py	Removes snapshot-specific CLI options from pytest configuration.
pyproject.toml	Removes snapshot-related pytest marker registrations.
.github/workflows/db_sync_full_sync.yaml	Removes run_only_sync_test toggle and runs only node_sync/db_sync marked tests.

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