The change stream pipeline microbenchmarks previously replaced the head of the pipeline (the oplog match stage) with a synthetic queue plus a standalone match stage. That measured a partially mocked topology: it never exercised the cursor stage's batching, resume-tracking, catalog- resource lifecycle, or the fact that the oplog predicate is evaluated inside the inner executor.

This change modifies the benchmarks so that they keep the real oplog match stage and reuse its compiled predicate as the filter for an in-memory PlanExecutor that generates oplog entries on the fly, attached through a real $cursor stage.
This matches production topology without writing to or reading from a physical oplog.

The reusable cursor mocks were taken from the existing test document_source_cursor_test.cpp and extracted into a shared test helper that can be reused the test and the microbenchmarks.