This PR makes small refactors and minor efficiency improvements in the replication oplog fetcher, primarily around document validation, batch materialization, and const-correctness.

Changes:

• Precompute the `gReduceMajorityWriteLatency` feature-flag check once per `validateDocuments()` call instead of per document.
• Reserve batch vector capacity based on the cursor’s remaining objects in the current batch to reduce reallocations.
• Mark `_checkRemoteOplogStart()` / `_checkTooStaleToSyncFromSource()` as const.