feat: add route pool size negotiation (Gap 13.3)

Add NegotiatePoolSize static method and NegotiatedPoolSize property to
RouteConnection, and ConfiguredPoolSize / GetEffectivePoolSize to
RouteManager. Includes 14 tests covering negotiation semantics, backward
compatibility (zero means no pooling), default state, and deterministic
pool index computation.

src/NATS.Server/Routes/RouteConnection.cs

@@ -22,6 +22,36 @@ public sealed class RouteConnection(Socket socket) : IAsyncDisposable
     /// for a given account. See <see cref="RouteManager.ComputeRoutePoolIdx"/>.
     /// </summary>
     public int PoolIndex { get; set; }
+
+    /// <summary>
+    /// The pool size agreed upon during handshake negotiation with the remote peer.
+    /// Defaults to 0 (no pooling / pre-negotiation state). Set after handshake completes.
+    /// Go reference: server/route.go negotiateRoutePool.
+    /// </summary>
+    public int NegotiatedPoolSize { get; private set; }
+
+    /// <summary>
+    /// Negotiates the effective route pool size between local and remote peers.
+    /// Returns <c>Math.Min(localPoolSize, remotePoolSize)</c>, but returns 0 if
+    /// either side is 0 for backward compatibility with peers that do not support pooling.
+    /// Go reference: server/route.go negotiateRoutePool.
+    /// </summary>
+    public static int NegotiatePoolSize(int localPoolSize, int remotePoolSize)
+    {
+        if (localPoolSize == 0 || remotePoolSize == 0)
+            return 0;
+
+        return Math.Min(localPoolSize, remotePoolSize);
+    }
+
+    /// <summary>
+    /// Applies the result of pool size negotiation to this connection.
+    /// </summary>
+    internal void SetNegotiatedPoolSize(int negotiatedPoolSize)
+    {
+        NegotiatedPoolSize = negotiatedPoolSize;
+    }
+
     public Func<RemoteSubscription, Task>? RemoteSubscriptionReceived { get; set; }
     public Func<RouteMessage, Task>? RoutedMessageReceived { get; set; }