Description: Adds a small helper in explain_common that copies fields from a BSON reply into an output BSONObjBuilder, omitting protocol bookkeeping fields (ok, $clusterTime, operationTime) that should not appear inside explain output.

Rationale: When a shard acts as a subrouter (SERVER-42282) and embeds a downstream shard's explain reply into the top-level explain output, the raw reply contains protocol fields that belong at the outer command level, not inside stages[].$cursor or sibling structures. Centralizing the sanitization in one named helper avoids open-coding the field exclusion list at each call site.