In /etc/subversion/servers you are setting http-proxy-host, which has nothing to do with svn:// which connects to a different server usually running on port 3690 started by svnserve command.
If you have access to the server, you can setup svn+ssh:// as explained here.
Update: You could also try using connect-tunnel, which uses your HTTPS proxy server to tunnel connections:
connect-tunnel -P proxy.company.com:8080 -T 10234:svn.example.com:3690
Then you would use
svn checkout svn://localhost:10234/path/to/trunk