test(security): fix Logout_clears_the_cookie

Two pre-existing test bugs surfaced by the auth-alignment branch:
 - Test wanted the 204/JSON contract but never sent Accept:
   application/json — endpoint correctly returned 302 (form POST).
 - Cookie-name assertion still used OtOpcUa.Auth= (now
   ZB.MOM.WW.OtOpcUa.Auth= since the Task 1 default change).

Endpoint behavior is intentional and untouched.

tests/Server/ZB.MOM.WW.OtOpcUa.Security.Tests/AuthEndpointsIntegrationTests.cs

@@ -184,12 +184,13 @@ public sealed class AuthEndpointsIntegrationTests : IAsyncLifetime
         loginResponse.EnsureSuccessStatusCode();
 
         var logoutReq = new HttpRequestMessage(HttpMethod.Post, "/auth/logout");
+        logoutReq.Headers.Accept.ParseAdd("application/json");
         AttachCookies(logoutReq, loginResponse);
         var response = await client.SendAsync(logoutReq, Ct);
         response.StatusCode.ShouldBe(HttpStatusCode.NoContent);
 
         response.Headers.GetValues("Set-Cookie")
-            .ShouldContain(c => c.StartsWith("OtOpcUa.Auth=") && c.Contains("expires=", StringComparison.OrdinalIgnoreCase));
+            .ShouldContain(c => c.StartsWith("ZB.MOM.WW.OtOpcUa.Auth=") && c.Contains("expires=", StringComparison.OrdinalIgnoreCase));
     }
 
     /// <summary>Anonymous browser GET of a protected route redirects to /login with a ReturnUrl.</summary>