MM-57873 Check user visibility when accepting channel invites for DMs (#27331)

* check user visibility when accepting channel invites for DMs

* stronger visibility checking for DM users

* check for correct remoteid for remote user in DM invite

* fix unit test

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Co-authored-by: Mattermost Build <build@mattermost.com>

server/platform/services/sharedchannel/service.go

@@ -55,6 +55,7 @@ type AppIface interface {
 	SendEphemeralPost(c request.CTX, userId string, post *model.Post) *model.Post
 	CreateChannelWithUser(c request.CTX, channel *model.Channel, userId string) (*model.Channel, *model.AppError)
 	GetOrCreateDirectChannel(c request.CTX, userId, otherUserId string, channelOptions ...model.ChannelOption) (*model.Channel, *model.AppError)
+	UserCanSeeOtherUser(c request.CTX, userID string, otherUserId string) (bool, *model.AppError)
 	AddUserToChannel(c request.CTX, user *model.User, channel *model.Channel, skipTeamMemberIntegrityCheck bool) (*model.ChannelMember, *model.AppError)
 	AddUserToTeamByTeamId(c request.CTX, teamId string, user *model.User) *model.AppError
 	PermanentDeleteChannel(c request.CTX, channel *model.Channel) *model.AppError