feat: add jetstream consumer api lifecycle

src/NATS.Server/JetStream/ConsumerManager.cs

@@ -0,0 +1,57 @@
+using System.Collections.Concurrent;
+using NATS.Server.JetStream.Api;
+using NATS.Server.JetStream.Models;
+using NATS.Server.JetStream.Storage;
+
+namespace NATS.Server.JetStream;
+
+public sealed class ConsumerManager
+{
+    private readonly ConcurrentDictionary<(string Stream, string Name), ConsumerHandle> _consumers = new();
+
+    public int ConsumerCount => _consumers.Count;
+
+    public JetStreamApiResponse CreateOrUpdate(string stream, ConsumerConfig config)
+    {
+        if (string.IsNullOrWhiteSpace(config.DurableName))
+            return JetStreamApiResponse.ErrorResponse(400, "durable name required");
+
+        var key = (stream, config.DurableName);
+        var handle = _consumers.AddOrUpdate(key,
+            _ => new ConsumerHandle(stream, config),
+            (_, existing) => existing with { Config = config });
+
+        return new JetStreamApiResponse
+        {
+            ConsumerInfo = new JetStreamConsumerInfo
+            {
+                Config = handle.Config,
+            },
+        };
+    }
+
+    public JetStreamApiResponse GetInfo(string stream, string durableName)
+    {
+        if (_consumers.TryGetValue((stream, durableName), out var handle))
+        {
+            return new JetStreamApiResponse
+            {
+                ConsumerInfo = new JetStreamConsumerInfo
+                {
+                    Config = handle.Config,
+                },
+            };
+        }
+
+        return JetStreamApiResponse.NotFound($"$JS.API.CONSUMER.INFO.{stream}.{durableName}");
+    }
+
+    public bool TryGet(string stream, string durableName, out ConsumerHandle handle)
+        => _consumers.TryGetValue((stream, durableName), out handle!);
+}
+
+public sealed record ConsumerHandle(string Stream, ConsumerConfig Config)
+{
+    public ulong NextSequence { get; set; } = 1;
+    public Queue<StoredMessage> Pending { get; } = new();
+}