What

Add `GUARDED_BY(_mutex)` to the state fields in `WaitForMajorityService` accessed without the mutex. Add `REQUIRES(_mutex)` to any private helper that reads them.

Files: `wait_for_majority_service.cpp` (line 136).

Resolves: SERVER-108874, SERVER-111265

Why

The service state is accessed from threads with different lock ownership. Because Coverity cannot be run locally, `GUARDED_BY` annotations are the practical way to confirm the fix is complete and to prevent these fields from being accessed without the lock in the future.