issue: #30531
cause get client from `shardClientMgr`, doesn't means query node is
unavailable. because of the ref counter policy in `shardClientMgr`,
which will clean the client, if no collection use qn as shard leader.
This PR fix that set node unreachable when get shard client failed.
Signed-off-by: Wei Liu <wei.liu@zilliz.com>