* MM-68838: ping restored plugin remote immediately on re-register
RegisterPluginForSharedChannels' restore branch updated the row but did
not call PingNow, leaving the restored remote offline until the next
pingLoop tick (up to PingFreq, default 1 minute). The new-connection
branch already calls PingNow; the restore branch now mirrors it so
sync attempts immediately after a plugin restart no longer fail with
"offline remote cluster".
* MM-68838: gob-encode error returns in apiRPCServer.ReceiveSharedChannelAttachmentSyncMsg
The apiRPCServer wrapper for ReceiveSharedChannelAttachmentSyncMsg
assigned the hook's error return directly to the gob-encoded response
struct. When the framework's App.ReceiveSharedChannelAttachmentSyncMsg
returned an error wrapped with %w (*fmt.wrapError, an unexported type),
gob refused to encode it and the RPC server broke the connection with
"type not registered for interface: fmt.wrapError".
Every subsequent plugin/server RPC call then returned the zero-value
response struct, causing plugins that dereferenced the nil returns to
crash.
Apply the existing encodableError() helper so the returned error
becomes a gob-safe ErrorString, matching every other apiRPCServer
method in this file.
