This came up when working on SERVER-123662.

Summary

The setQuerySettings command reply is inconsistent between inserting a new
query settings entry and updating an existing one. When featureFlagPQSBackfill
is enabled, an update reply omits representativeQuery, whereas an insert
reply (and the pre-flag behavior) includes it.

Background

Under featureFlagPQSBackfill the representative query is persisted separately
(in config.queryShapeRepresentativeQueries) and is intentionally cleared from
the querySettings cluster parameter entry.

• On the insert path, the code clears the representative query from the stored
  entry but re-attaches it to the reply object before returning — so the reply
  includes representativeQuery.
• On the update path, the code clears the representative query on the entry and
  then reuses that same cleared object as the reply — so the reply omits
  representativeQuery.

See SetQuerySettingsCommand::Invocation::setQuerySettings in
src/mongo/db/commands/query_cmd/query_settings_cmds.cpp.

Goal

The setQuerySettings reply should echo representativeQuery consistently
for both inserts and updates whenever a representative query was supplied,
regardless of featureFlagPQSBackfill.