Simplify KvStore.list() API to take optional prefix directly

Change the list() method signature from list(options: KvStoreListOptions)
to list(prefix?: KvKey) for a cleaner API. The KvStoreListOptions interface
is removed as it only contained a single prefix field.

When prefix is omitted or undefined, the method returns all entries in the
store.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>