Enforce FriendsOnly lobby privacy during handshake

[trende2001]

Summary

Adds server-side enforcement for LobbyPrivacy.FriendsOnly lobbies during the handshake phase. When a lobby is set to FriendsOnly, the host now validates the joining player's SteamId against its own Steam friend list and kicks non-friends with the message "This lobby is Friends Only."

Previously, non-friends could connect and remain in the lobby.

Motivation & Context

Players expect that setting their lobby to "Friends Only" actually prevents non-friends from joining. This change closes that gap at the engine level so game code doesn't need to reimplement it.

Implementation Details

• The check runs in On_Handshake_ClientInfo after the SteamId is resolved from the trusted SteamLobbyConnection, using Friend.IsFriend against the host's local friend list.
• The host always bypasses the check. Steam API failures fail closed (deny access).

Screenshots / Videos (if applicable)

Checklist

• Code follows existing style and conventions
• No unnecessary formatting or unrelated changes
• Public APIs are documented (if applicable)
• Unit tests added where applicable and all passing
• I’m okay with this PR being rejected or requested to change 🙂

[Copilot]

Pull request overview

This PR adds server-side enforcement for FriendsOnly lobby privacy during the handshake phase. When a lobby is configured with LobbyPrivacy.FriendsOnly, the host now validates the joining player's SteamId against its Steam friend list and kicks non-friends before they can fully connect.

Changes:

• Adds FriendsOnly validation in the On_Handshake_ClientInfo method after SteamId resolution
• Implements fail-closed error handling for Steam API failures
• Adds clear logging for both allowed and rejected connection attempts

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[Kaydax]

LGTM

[handsomematt]

Why is there a huge try catch around isFriend = joiningFriend.IsFriend;?

This is a good place to do this friend check, but your implementation can be a lot tighter.

[trende2001]

Why is there a huge try catch around isFriend = joiningFriend.IsFriend;?

This is a good place to do this friend check, but your implementation can be a lot tighter.

I made the implementation a lot more tighter without over defensive checks.