Summary:
`PgsqlVectorFilter` uses `FilteringIterator::has_filter` to check whether related iterator has filter to decide whether we could skip it.
But iterator is not yet initialised at this point, so it always returns `false`.
As result rows were filtered on the next step, and logic to determine whether there are more rows in the tablet were broken.

Fixed by introducing static function to check whether iterator will have filter.
Jira: DB-18097

Test Plan: PgVectorIndexTest.ManyRowsWithFilter/*

Reviewers: arybochkin

Reviewed By: arybochkin

Subscribers: ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D46284