fix(dashboard): make dashboard auth cookie name configurable

The dashboard auth cookie name was hardcoded to the constant
DashboardAuthenticationDefaults.CookieName (MxGatewayDashboard). Browser
cookies are scoped by host+path but NOT by port, so two gateway instances
sharing a hostname would clobber each other's dashboard session under the
shared name.

Add DashboardOptions.CookieName (MxGateway:Dashboard:CookieName); null/blank
keeps the canonical default. Applied in the existing dashboard cookie
PostConfigure (runs after the inline AddCookie default, so it wins). Behaviour
is unchanged when unset. Adds a Tests case for the override.

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Dashboard/DashboardCookieOptionsTests.cs

@@ -49,4 +49,23 @@ public sealed class DashboardCookieOptionsTests
 
         Assert.Equal(CookieSecurePolicy.SameAsRequest, options.Cookie.SecurePolicy);
     }
+
+    /// <summary>
+    /// Verifies that <c>MxGateway:Dashboard:CookieName</c> overrides the dashboard auth
+    /// cookie name, so a gateway instance sharing a hostname with another can be given a
+    /// distinct name (browser cookies are scoped by host+path, not port).
+    /// </summary>
+    [Fact]
+    public async Task Build_WithCookieNameOverride_UsesConfiguredName()
+    {
+        await using WebApplication app = GatewayApplication.Build(
+            ["--MxGateway:Dashboard:CookieName=MxGatewayDashboard.env2"]);
+        IOptionsMonitor<CookieAuthenticationOptions> optionsMonitor = app.Services
+            .GetRequiredService<IOptionsMonitor<CookieAuthenticationOptions>>();
+
+        CookieAuthenticationOptions options = optionsMonitor.Get(
+            DashboardAuthenticationDefaults.AuthenticationScheme);
+
+        Assert.Equal("MxGatewayDashboard.env2", options.Cookie.Name);
+    }
 }