test: assert expired snapshot ids in expire_snapshots tests

[anxkhn]

Rationale for this change

test_expire_unprotected_snapshot and test_expire_snapshots_by_ids in
tests/table/test_expire_snapshots.py do not actually exercise the expiration
logic. Both tests mock the catalog and hardcode the commit_table response
metadata to snapshots=[KEEP_SNAPSHOT]. After the commit, Table._do_commit
sets self.metadata = response.metadata, so table_v2.metadata.snapshots
becomes that canned value regardless of which ids ExpireSnapshots actually
selected. The post-conditions then only re-read the mock:

• assert EXPIRE_SNAPSHOT not in remaining_snapshots compares an int against a
  list of a different int, which is always true.
• assert len(table_v2.metadata.snapshots) == 1 just reads the hardcoded
  single-element list.

As a result, both tests still pass even when ExpireSnapshots.by_id / by_ids
are neutered to no-ops (nothing is expired), so a regression that selected the
wrong snapshot ids, or none, would not be caught.

This mirrors the assertion style already used correctly in the same file by
test_expire_snapshots_by_timestamp_skips_protected, which inspects the
RemoveSnapshotsUpdate sent to commit_table instead of the mocked response.

Are these changes tested?

Yes; this change is entirely to tests. It replaces the assertions in the two
tests above with an inspection of the RemoveSnapshotsUpdate.snapshot_ids
actually passed to commit_table (via call_args), comparing as a set because
the update's ids are built from a set.

• python -m pytest tests/table/test_expire_snapshots.py -> 9 passed.
• To confirm the assertions now have teeth: with ExpireSnapshots.by_id /
  by_ids temporarily stubbed to no-ops, both tests now fail (the captured
  snapshot_ids set is empty), whereas before this change they still passed.

Are there any user-facing changes?

No. Test-only change; no library code is modified.

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Notes for maintainers (optional comment, not part of the template)

• No new tests are added; this tightens two existing ones so they assert the
  real expiration result. The other tests in the file are unchanged.
• The inline call_args inspection is duplicated across three tests now
  (including the pre-existing ..._skips_protected). I kept it inline to match
  the established in-file pattern rather than introduce a helper in this PR; happy
  to factor it out if preferred.