Fix duplicate neighbors race between HNSW INSERT and VACUUM (#1010)

This fixes a race condition where concurrent INSERT and VACUUM
processes can write the same element into a neighbor array twice,
causing duplicate neighbors and wasting fixed node out-degree capacity.

The issue happens because INSERT performs blind writes. If a concurrent
VACUUM discovers the newly inserted element via its first bridge edge,
VACUUM safely writes the connection first. When INSERT resumes, it blindly
overwrites it. This change removes the 'checkExisting' flag, making sure that
existing neighbors are always checked before inserting a new node.