Overview

On a sharded cluster, running explain on an extension $search / $searchMeta pipeline that
contains $unionWith + $lookup fails to serialize the explain response:

BSON field 'VectorClockComponents.explainVersion' is a duplicate field
code: 40413 \(IDLDuplicateField\)

The explainVersion field is written twice into the explain/response BSON, tripping IDL
duplicate-field validation. This is an explain-serialization bug (distinct from SERVER-131024, which
adapted explain test expectations to the extension stage names).

Steps to reproduce

resmoke.py run \-\-suites=with\_mongot\_extension\_sharded\_collections 
  jstests/with\_mongot/e2e/explain\_execution\_stats/search\_union\_with\_lookup\_explain.js

Hypothesis

The router and shard both contribute an explainVersion (or a VectorClock component is appended
twice) when merging explain output for a subpipeline extension search stage.

Affected tests (currently excluded in with_mongot_extension override)

* jstests/with_mongot/e2e/explain_execution_stats/search_union_with_lookup_explain.js
* jstests/with_mongot/e2e/explain_execution_stats/search_meta_union_with_lookup_explain.js

Acceptance

Remove the two exclusions tagged with this ticket in the with_mongot_extension override.