Summary:
It could happen that tserver was restarted after vector index was flushed, but regular db was not flushed.
In this case we could get into situation where reverse index (vector id => ybctid) is missing in regular db.
This diff changes logic to handle this situation correctly.
Jira: DB-15413
Original commit: 6fa451ba4e84f965aff61a4feb4aaa90731cd69e / D41972

Test Plan: PgVectorIndexTest.BootstrapFlushedVectorIndexes/*

Reviewers: arybochkin, aleksandr.ponomarenko, slingam

Reviewed By: slingam

Subscribers: yql, ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D42094