[DocDB][PgVector] Vector Index ANN query returns empty results after rolling restart when DocDB flushed but vector LSM was not

[archit-rastogi]

Jira Link: DB-22415

Description

Description:
Exact kNN OK; ybhnsw returns []; EXPLAIN still chooses HNSW.

Cause:
Vector index persists only a backfill_done placeholder (file/index NULL).
After restart, bootstrap skips WAL ops already in RocksDB and does not rebuild VI.

Test: https://itest-system.dev.yugabyte.com/test/149c3c28-eb6d-41e7-abea-c87df64e3714

Issue Type

kind/bug

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