feat: enforce jetstream retention and limits

src/NATS.Server/JetStream/Storage/MemStore.cs

@@ -54,4 +54,16 @@ public sealed class MemStore : IStreamStore
             });
         }
     }
+
+    public void TrimToMaxMessages(ulong maxMessages)
+    {
+        lock (_gate)
+        {
+            while ((ulong)_messages.Count > maxMessages)
+            {
+                var first = _messages.Keys.Min();
+                _messages.Remove(first);
+            }
+        }
+    }
 }