Summary:
Commit b5b495dbd9b4ac6ec197705f89c795d9510c799e incorrectly translates
some logic, causing system and copartition secondary index scans to lose
the single-RPC optimization to embed a table and index scan together in
the same RPC.  Fix the logic and add tests to cover the cases.
Jira: DB-16789

Test Plan:
On Almalinux 8:

    ./yb_build.sh fastdebug --gcc11 daemons initdb \
      --cxx-test pgwrapper_pg_libpq-test \
      --gtest_filter PgLibPqTest.Embedded\*

Close: #27294

Reviewers: sanketh

Reviewed By: sanketh

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D44085