fix: compile the lance tests after the compacted_sstables rename

[wombatu-kun]

main does not compile its tests. cargo test -p lance --lib stops at three uses of a field that no longer exists:

error[E0559]: variant `dataset::transaction::Operation::Update` has no field named `merged_generations`
    --> rust/lance/src/dataset/transaction.rs:5453:17
     |
5453 |                 merged_generations: vec![],
     |                 ^^^^^^^^^^^^^^^^^^ `dataset::transaction::Operation::Update` does not have this field
     |
     = note: available fields are: `compacted_sstables`

#7957 renamed that field to compacted_sstables in Operation::Update and in pb::transaction::Update. #7432 was branched before it and its tests still name the old field, so the merge produced code that no compiler had seen: neither branch was wrong on its own base, and git had no textual conflict to report.

All three sites pass an empty list, so they take the new name unchanged. With it, cargo test -p lance --lib dataset::transaction::tests is back to 64 passed.

[lance-gatekeeper]

✅ Gate recommendation: approve.

The change updates the three stale test initializers to the current Rust and protobuf field name. It restores the transaction test suite without changing production behavior or wire semantics.

[wombatu-kun]

@Xuanwo could you take a look when you have a moment? Test-only, three lines.

Until it lands, CI on every new PR fails for a reason unrelated to that PR: the checks build against the merge with main, and cargo test -p lance --lib does not compile there. lance-gatekeeper has already recommended approval.