What this PR does / why we need it:

RedisOnlineStore implements full async support (online_read_async,
online_write_batch_async, _get_client_async using redis.asyncio), but
does not override the async_supported property from the base OnlineStore
class. The default returns SupportedAsyncMethods(read=False, write=False),
so the feature server never uses the async path — falling back to
run_in_threadpool(sync) on every request.

Which issue(s) this PR fixes:

Fixes #6437