mgr/cephadm: write agent required_files on SSH reconfig

SSH agent reconfig returned success and restarted the unit but did not
rewrite agent.json, so after mgr failover agents kept a stale target_ip.
Reuse the same required_files write path as HTTP config push, with tests.

Fixes: https://tracker.ceph.com/issues/78887
Signed-off-by: Ashwin M. Joshi <ashjosh1@in.ibm.com>