SQL Server's DbUpdateException for one duplicate names every entry in the statement batch, so detaching
ex.Entries dropped the whole flush. On a row-level rejection the writer now asks the database which of the
batch's EventIds it already holds, drops exactly those, re-saves the rest, and falls back to row-by-row only
if a non-duplicate row is still refused. Fresh context + entities per attempt. Pinned against the real
central SQL Server (new SqlServerTestDatabase harness) and a faithful whole-batch interceptor.
Refs #733
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty