Update RefreshSignInAsync documentation and fix null checks

- Updated XML documentation to clearly state user must be signed in
- Added explicit null check for auth result at the beginning
- Removed unnecessary null-conditional operators after auth.Succeeded check
- All existing tests pass

Co-authored-by: halter73 <[email protected]>

Author: Copilot

src/Identity/Core/src/SignInManager.cs

@@ -165,10 +165,14 @@ public virtual async Task<bool> CanSignInAsync(TUser user)
     }
 
     /// <summary>
-    /// Signs in the specified <paramref name="user"/>, whilst preserving the existing
+    /// Refreshes the sign-in for the specified <paramref name="user"/>, whilst preserving the existing
     /// AuthenticationProperties of the current signed-in user like rememberMe, as an asynchronous operation.
     /// </summary>
-    /// <param name="user">The user to sign-in.</param>
+    /// <remarks>
+    /// The user must already be signed in, and the user ID must match the currently authenticated user.
+    /// If the user is not signed in, use <see cref="SignInAsync(TUser, bool, string)"/> instead.
+    /// </remarks>
+    /// <param name="user">The user to refresh the sign-in for.</param>
     /// <returns>The task object representing the asynchronous operation.</returns>
     public virtual async Task RefreshSignInAsync(TUser user)
     {
@@ -189,6 +193,12 @@ public virtual async Task RefreshSignInAsync(TUser user)
     private async Task<(bool success, bool? isPersistent)> RefreshSignInCoreAsync(TUser user)
     {
         var auth = await Context.AuthenticateAsync(AuthenticationScheme);
+        if (auth == null)
+        {
+            Logger.LogError("RefreshSignInAsync prevented because authentication result is null.");
+            return (false, null);
+        }
+
         if (!auth.Succeeded || auth.Principal?.Identity?.IsAuthenticated != true)
         {
             Logger.LogError("RefreshSignInAsync prevented because the user is not currently authenticated. Use SignInAsync instead for initial sign in.");
@@ -204,8 +214,8 @@ public virtual async Task RefreshSignInAsync(TUser user)
         }
 
         IList<Claim> claims = Array.Empty<Claim>();
-        var authenticationMethod = auth.Principal?.FindFirst(ClaimTypes.AuthenticationMethod);
-        var amr = auth.Principal?.FindFirst("amr");
+        var authenticationMethod = auth.Principal.FindFirst(ClaimTypes.AuthenticationMethod);
+        var amr = auth.Principal.FindFirst("amr");
 
         if (authenticationMethod != null || amr != null)
         {
@@ -221,7 +231,7 @@ public virtual async Task RefreshSignInAsync(TUser user)
         }
 
         await SignInWithClaimsAsync(user, auth.Properties, claims);
-        return (true, auth.Properties?.IsPersistent ?? false);
+        return (true, auth.Properties.IsPersistent);
     }
 
     /// <summary>