BatchedEnrichmentStage buffers events in two std::deque members (the pre-enrichment input buffer and the enriched output buffer). Both are bounded by the stage's configured limits (max input events and max output bytes), so they can use a fixed-capacity ring buffer instead of a std::deque that reallocates on every push/pop. Change-stream and $search enrichment is a hot path, so cutting that allocation churn is worthwhile.

Replace the two std::deque members in src/mongo/db/exec/agg/batched_enrichment_stage.h with the RingBuffer primitive, sizing capacity from the stage's limits.

Behaviour-preserving: the existing BatchedEnrichmentStageTest suite is the regression bar.

Depends on SERVER-129348 (the RingBuffer primitive).