What

Replace `readLock()` with `writeLock()` for all `SharedPromise::emplaceValue` and `SharedPromise::setError` calls on the promise at line 2083 of `replication_coordinator_impl.h`.

Files: `replication_coordinator_impl.h` (line 2083).

Resolves: SERVER-108524

Why

`SharedPromise` allows only one fulfillment; using `readLock()` at the fulfillment site permits concurrent writers, violating the single-write contract and producing a data race. Replacing it with `writeLock()` restores mutual exclusion. Because Coverity cannot be run locally, the type change is the verifiable proof — Coverity's model for `SharedPromise::writeLock` enforces the single-writer constraint.