`large_txn_basic.js` and `prepare_txn_oplog_application_basic.js` run `prepareTransaction` but were not tagged with `featureFlagPreparedTransactionsPreciseCheckpoints`. When the flag is off (the nightly `disagg_storage` task on `atlas-amazon2023-arm64` / `atlas-ubuntu2204-arm64`), `prepareTransaction` is rejected with `NotImplemented` "Disaggregated Storage does not support cross-shard transactions" and the tests fail. See BF-44620.

`DisaggregatedPersistenceProvider::supportsCrossShardTransactions()` returns the flag value, and `prepareTransaction` uasserts when it is false. The other prepared transaction tests in `disagg_storage` (`recover_prepare_*.js`) already carry this tag, so resmoke only runs them when the flag is enabled. This change adds the tag to the two tests that were missing it.