The registration of `QuarantinedMediaStream` in
`ReplicationCommandHandler._streams_to_replicate` was missed when the
stream was added, so an instance configured as the
quarantined_media_changes stream writer never sent RDATA/POSITION for it
unless it was the main process.
Also add the stream to the `instance_map` config validation.
Stream was introduced in
https://github.com/element-hq/synapse/pull/19558
Fixes https://github.com/element-hq/synapse/issues/20080
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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>