feat: complete remaining jetstream parity implementation plan

src/NATS.Server/JetStream/Api/Handlers/DirectApiHandlers.cs

@@ -0,0 +1,61 @@
+using System.Text;
+using System.Text.Json;
+
+namespace NATS.Server.JetStream.Api.Handlers;
+
+public static class DirectApiHandlers
+{
+    private const string Prefix = JetStreamApiSubjects.DirectGet;
+
+    public static JetStreamApiResponse HandleGet(string subject, ReadOnlySpan<byte> payload, StreamManager streamManager)
+    {
+        var streamName = ExtractTrailingToken(subject, Prefix);
+        if (streamName == null)
+            return JetStreamApiResponse.NotFound(subject);
+
+        var sequence = ParseSequence(payload);
+        if (sequence == 0)
+            return JetStreamApiResponse.ErrorResponse(400, "sequence required");
+
+        var message = streamManager.GetMessage(streamName, sequence);
+        if (message == null)
+            return JetStreamApiResponse.NotFound(subject);
+
+        return new JetStreamApiResponse
+        {
+            DirectMessage = new JetStreamDirectMessage
+            {
+                Sequence = message.Sequence,
+                Subject = message.Subject,
+                Payload = Encoding.UTF8.GetString(message.Payload.Span),
+            },
+        };
+    }
+
+    private static string? ExtractTrailingToken(string subject, string prefix)
+    {
+        if (!subject.StartsWith(prefix, StringComparison.Ordinal))
+            return null;
+
+        var token = subject[prefix.Length..].Trim();
+        return token.Length == 0 ? null : token;
+    }
+
+    private static ulong ParseSequence(ReadOnlySpan<byte> payload)
+    {
+        if (payload.IsEmpty)
+            return 0;
+
+        try
+        {
+            using var doc = JsonDocument.Parse(payload.ToArray());
+            if (doc.RootElement.TryGetProperty("seq", out var seqEl) && seqEl.TryGetUInt64(out var sequence))
+                return sequence;
+        }
+        catch (JsonException)
+        {
+        }
+
+        return 0;
+    }
+}