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.Server/Configuration/DashboardOptions.cs

@@ -21,6 +21,17 @@ public sealed class DashboardOptions
     /// </summary>
     public bool RequireHttpsCookie { get; init; } = true;
 
+    /// <summary>
+    /// Dashboard auth cookie name. When null/blank (the default) the canonical
+    /// <see cref="ZB.MOM.WW.MxGateway.Server.Dashboard.DashboardAuthenticationDefaults.CookieName"/>
+    /// is used. Override it (<c>MxGateway:Dashboard:CookieName</c>) to give a distinct name to a
+    /// gateway that shares a hostname with another gateway instance — browser cookies are scoped
+    /// by host+path but NOT by port, so two instances on the same host would otherwise clobber
+    /// each other's dashboard session under a shared cookie name. Changing this signs out
+    /// existing dashboard sessions on next deploy.
+    /// </summary>
+    public string? CookieName { get; init; }
+
     /// <summary>Gets the dashboard snapshot update interval in milliseconds.</summary>
     public int SnapshotIntervalMilliseconds { get; init; } = 1_000;