Context

Follow-up from review discussion on PR #58187: https://github.com/10gen/mongo/pull/58187#discussion_r3560740611

IncrementalFeatureRolloutContext::mutableShardServerDefaultTemplate() (src/mongo/db/feature_flag.cpp) returns a static template that is only meaningful on shard servers — it's populated by initShardServerDefaultTemplate() during shard-server startup and cloned in installForRequestWithoutIfrFlags() when clusterRole.has(ClusterRole::ShardServer).

Nothing currently prevents this accessor from being called on a mongos (router-only) process, where the template is never initialized. Such a call would silently return a default-constructed, empty IncrementalFeatureRolloutContext rather than failing loudly.

Suggestion

Add a tassert in mutableShardServerDefaultTemplate() (or at its call site) guarding that the current process has the shard server cluster role, so a mistaken router-side access fails fast instead of silently returning bad data.

Reviewer (mjrb) flagged this as optional/annoying to implement, hence filing as a low-priority follow-up rather than blocking the original PR.