Summary:
Add the `VectorIndex_Search` wait event, under a new `VectorIndex` ASH class, so that pgvector / ybhnsw approximate nearest neighbor searches are visible in Active Session History.
Instrument `VectorLSM::Search` with `SCOPED_WAIT_STATUS`, covering both the in-memory insert registry search and the per-chunk index searches.
Classify the event as `kCpu`, since HNSW graph traversal and distance computation dominate the search.
Set the tablet id on the wait state aux, which the read RPC handler already populates.

Test Plan:
./yb_build.sh debug --cxx-test pg_ash-test --gtest_filter PgAshTest.VectorIndexSearch

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_automated · Claude Code (Opus 4.8)_

Reviewers: asaha

Reviewed By: asaha

Subscribers: yql, ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D55129