Context

On a sharded cluster the SBE/Express primary returns kNotHandled when the post-image no longer lives
on the shard observing the update — caused by moveChunk, moveCollection, reshardCollection, etc.
PrimaryWithFallback then calls the Aggregation fallback. This is the genuine production trigger for
the aggregation cell and the only place the primary->fallback handoff is observable; it cannot be
reached on a replset (as lookups are always local).

Scope

Test-only (the executor wiring is done in the per-engine tickets). A dedicated test with its own
ShardingTest that controls placement and asserts metric deltas after each DDL op.

Acceptance / test matrix

New jstests/sharding/query/change_streams/change_stream_metrics_update_lookup.js. For each of
moveChunk, moveCollection, reshardCollection (and any op that relocates the post-image or changes the
UUID): place a doc, open a collection change stream, perform the op so the post-image becomes remote
or the UUID changes, update the doc, then assert (helpers already exist):

• updateLookup.{express, sbe}.notHandled +1 (primary declined), and
• updateLookup.aggregation.found +1 (fallback handled it).
  Also assert the deleted/absent post-image case routes to aggregation.notFound.