fix(ssh): forward custom DERP map to ProxyCommand

[zukka77]

Fix tailcat ssh with custom --derpmap-url

Summary

Fixes an issue where tailcat ssh could not connect using a short, RegionID-based token when --derpmap-url specified a custom DERP map.

tailcat ssh launches OpenSSH with a ProxyCommand that invokes tailcat again. That internal tailcat command forwarded --key, but not --derpmap-url, so it fell back to the default DERP map and could not resolve custom
RegionIDs.

Change

When a non-default DERP map URL is configured, the generated ProxyCommand now includes it:

tailcat --key=... --derpmap-url=https://derp.example.com/derpmap.json 22

Default-map behavior remains unchanged.

Test coverage

Added unit coverage verifying that:

• A custom DERP map URL is included in the SSH ProxyCommand.
• The default DERP map URL is not unnecessarily included.