[Bug]: VersionedDatabaseTaskStore get() and delete() fail with "no such table" when initialize() was not called #1356

What happened?

VersionedDatabaseTaskStore (src/a2a/server/cluster/database_task_store.py, added in #1281) creates its tables lazily, like DatabaseTaskStore: save() calls self._db._ensure_initialized() and then self._ensure_cluster_tables() before touching the database. get() and delete() only initialize the underlying DatabaseTaskStore, so on that path the task_versions table is never created.

If the store is used without calling initialize() first and the first operation is a get() or a delete(), the query on task_versions fails instead of returning None for a missing task:

store = VersionedDatabaseTaskStore(engine=create_async_engine('sqlite+aiosqlite:///:memory:'))

await store.get('missing', ServerCallContext())
# sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) no such table: task_versions

await store.delete('missing', ServerCallContext())
# sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) no such table: task_versions

The cluster sample and the existing tests call initialize() explicitly, so this only shows up when relying on the lazy initialization that save() and DatabaseTaskStore already provide.

Suggested fix: call self._ensure_cluster_tables() in get() and delete() after initializing the underlying store, as save() does.

I have a fix ready and I'm opening a PR alongside this issue.

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