What

Add `GUARDED_BY(_mutex)` to the state fields in `OplogBufferBatchedQueue` and `OplogBufferBlockingQueue` accessed without the mutex. Add `REQUIRES(_mutex)` to the enqueue/dequeue paths that bypass the lock.

Files: `oplog_buffer_batched_queue.cpp` (line 44), `oplog_buffer_blocking_queue.cpp` (line 84).

Resolves: SERVER-112310, SERVER-112388

Why

Both buffer implementations share the same interface and the same access pattern — state written under lock in the hot path but readable from other threads. Because Coverity cannot be run locally, `GUARDED_BY` annotations are the practical way to confirm the fix is complete and to catch regressions at build time.