Summary

Make the optimized updateLookup path safely disablable on a live cursor. When
featureFlagChangeStreamOptimizedUpdateLookup is turned off, any cursor still on the optimized
path is killed at its next getMore and the stream resumes on the legacy Aggregation path.

Scope

• Add a getMore precondition seam in getmore_cmd.cpp, mirroring the existing change-stream
  readPreference enforcement check, that detects a cursor on the optimized path while the flag is
  now off and surfaces a resumable error (kill-and-resume).
• New cursors observe the current flag value at construction (handled by the factory gate in the
  Express-wiring ticket); this ticket covers the transition for already-open cursors.

Files

• src/mongo/db/commands/query_cmd/getmore_cmd.cpp

Acceptance criteria

• A noPassthrough jstest opens an optimized change stream, turns the flag off, and asserts the
  next getMore kills the cursor and the stream resumes on the legacy Aggregation path.
• The test also asserts new cursors track the current flag value.

Dependencies

Depends on SERVER-117887 and SERVER-128414 (Express wiring) – there must be an optimized
path to retire.