Summary:
Follow up on 68f7de82addd7eaed4b30db0b1c5d8c1dc590dc0 / D39430 to
address outstanding review comments.
Jira: DB-13202

Test Plan: ./yb_build.sh --cxx-test pgwrapper_pg_index_backfill-test --gtest_filter PgIndexBackfillTest.PhantomIdxEntry

Reviewers: jason

Reviewed By: jason

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D39810