Description: Introduces CommandHelpers::runWithCommandErrorHandling, a generic wrapper that runs a function inside command dispatch's standard error-handling pattern (rethrow StaleConfig, convert other DBExceptions into a command error reply via appendCommandStatusNoThrow) and returns the resulting BSONObj.

Refactors count_cmd.cpp::runCountAsAgg to call runAggregate directly through this helper, replacing the current CommandHelpers::runCommandDirectly path that was constructing an OpMsgRequest solely to reuse its error-handling shell.

Rationale: The existing runCountAsAgg path serializes the aggregate command back into BSON and reparses it through command dispatch purely for error handling — wasteful and indirect. Extracting the error-handling pattern into a small reusable helper lets count dispatch aggregate directly. The helper also unblocks upcoming subrouter work (SERVER-42282) which needs the same error-handling shell around internal synchronous calls.