feat: add jetstream publish preconditions and dedupe

src/NATS.Server/JetStream/Publish/PublishPreconditions.cs

@@ -0,0 +1,25 @@
+using System.Collections.Concurrent;
+
+namespace NATS.Server.JetStream.Publish;
+
+public sealed class PublishPreconditions
+{
+    private readonly ConcurrentDictionary<string, ulong> _dedupe = new(StringComparer.Ordinal);
+
+    public bool IsDuplicate(string? msgId, out ulong existingSequence)
+    {
+        existingSequence = 0;
+        if (string.IsNullOrEmpty(msgId))
+            return false;
+
+        return _dedupe.TryGetValue(msgId, out existingSequence);
+    }
+
+    public void Record(string? msgId, ulong sequence)
+    {
+        if (string.IsNullOrEmpty(msgId))
+            return;
+
+        _dedupe[msgId] = sequence;
+    }
+}