refactor(auth): drop null-guard on dev steam ticket log; add test fixture doc

ISteamServer contract forbids null tickets (prod impl and sole caller both assume non-null),
so the dev bypass no longer needs the ?. / ?? 0 defensive form. Also adds a class-level XML
doc summary to DevAlwaysValidSteamServerTests matching the style of other fixtures in the suite.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Services/DevAlwaysValidSteamServer.cs

@@ -21,7 +21,7 @@ public sealed class DevAlwaysValidSteamServer : ISteamServer
     {
         _logger.LogWarning(
             "DEV Steam bypass: accepting ticket for steamId {SteamId} WITHOUT validation (ticketLen={Len}).",
-            steamId, ticket?.Length ?? 0);
+            steamId, ticket.Length);
         return true;
     }