Make `KvStore.list()` required in 2.0.0

[dahlia]

In #498, we are adding an optional list() method to the KvStore interface for Fedify 1.10.0. This gives existing KvStore implementations time to add support for prefix scanning.

In Fedify 2.0.0, the list() method should become a required part of the KvStore interface. This simplifies the interface contract and allows Fedify internals to rely on list() being available without fallback logic.

The migration path is straightforward: implementers have the entire 1.x cycle to add list() support, and the change will be documented as a breaking change in the 2.0.0 release notes.

Related issues

• Add optional list() operation to KvStore #498 (adds optional list() in 1.10.0)