Add regression test for DenoKvStore.list() exact prefix key

This test verifies that when an exact prefix key exists alongside child
keys, it is yielded exactly once (not duplicated or missing).

Note: Deno KV's native list() does NOT include exact prefix matches,
only strictly longer keys. The explicit check in DenoKvStore.list() is
necessary to include the exact prefix key in results.

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