Root Cause:
SERVER-108482 correctly removed the isNotPrimaryError retry from kNotIdempotent in Shard::remoteIsRetriableError, which had previously masked config-stepdown errors by silently retrying setDefaultRWConcern at the C++ layer before they reached the JS test layer. With that safety net gone, when the config primary steps down during write-concern satisfaction of setDefaultRWConcern, the response surfaces as writeConcernError: "code: 6, HostUnreachable, "operation was interrupted" ": an error the suite's retry override (implicitly_retry_on_config_stepdowns.js) was never updated to handle.

Fix (test-only):
Add ErrorCodes.HostUnreachable to isRetryableException in jstests/libs/override_methods/implicitly_retry_on_config_stepdowns.js, consistent with the precedent set in implicitly_retry_on_shard_transition_errors.js and the suite's own _stopBalancerForInit override. The server behavior is correct; no server-side change is required.