What

Add `GUARDED_BY(_mutex)` to the member fields in `OplogFetcher` accessed without the mutex. Add `REQUIRES(_mutex)` to private helpers that read or write them. Fix all callers that must acquire the lock before invoking those helpers.

Files: `oplog_fetcher.cpp` (lines 361, 507, 720).

Resolves: SERVER-110035, SERVER-111631, SERVER-112316

Why

Oplog fetcher state is accessed across executor-callback boundaries where lock ownership is ambiguous. Because Coverity cannot be run locally, `GUARDED_BY` annotations give the compiler the same visibility as the analyzer — a clean build proves the locking contract is satisfied for these fields, and regressions are caught at build time.