ext: HttpContext.GetUdid() over SID-mapping service

Mirrors how the translation middleware resolves the per-request UDID;
needed by ToolController.Signup and the SteamSession find-or-link
branch.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Extensions/HttpContextExtensions.cs

@@ -1,11 +1,13 @@
 using SVSim.Database.Models;
+using SVSim.EmulatedEntrypoint.Constants;
+using SVSim.EmulatedEntrypoint.Services;
 
 namespace SVSim.EmulatedEntrypoint.Extensions;
 
 public static class HttpContextExtensions
 {
     private const string ViewerItemName = "SVSimViewer";
-    
+
     public static Viewer? GetViewer(this HttpContext context)
     {
         if (context.Items.TryGetValue(ViewerItemName, out object? viewer))
@@ -21,4 +23,18 @@ public static class HttpContextExtensions
         context.Items[ViewerItemName] = viewer;
         return viewer;
     }
-}
+
+    /// <summary>
+    /// Resolves the client's UDID for this request by looking up the SID header in the
+    /// in-memory SID→UDID dict that <see cref="Middlewares.SessionidMappingMiddleware"/>
+    /// populates from the UDID header. Returns null when the SID isn't mapped (e.g. the
+    /// request didn't carry a UDID header at all, or carried an undecodable one).
+    /// </summary>
+    public static Guid? GetUdid(this HttpContext context)
+    {
+        string? sid = context.Request.Headers[NetworkConstants.SessionIdHeaderName];
+        if (sid is null) return null;
+        var sessionService = context.RequestServices.GetService<ShadowverseSessionService>();
+        return sessionService?.GetUdidFromSessionId(sid);
+    }
+}