Fix JetStream API serialization with typed results

Replace JsonDocument-based responses with strongly-typed `NatsJSApiResult<T>`
for improved type safety and error handling. Added new deserialization logic
and tests to cover valid responses, errors, and edge cases like empty buffers.

src/NATS.Client.JetStream/Internal/NatsJSApiResult.cs

@@ -0,0 +1,59 @@
+using System.Runtime.CompilerServices;
+using NATS.Client.JetStream.Models;
+
+namespace NATS.Client.JetStream.Internal;
+
+internal readonly struct NatsJSApiResult<T>
+{
+    private readonly T? _value;
+    private readonly ApiError? _error;
+    private readonly Exception? _exception;
+
+    public NatsJSApiResult(T value)
+    {
+        _value = value;
+        _error = null;
+        _exception = null;
+    }
+
+    public NatsJSApiResult(ApiError error)
+    {
+        _value = default;
+        _error = error;
+        _exception = null;
+    }
+
+    public NatsJSApiResult(Exception exception)
+    {
+        _value = default;
+        _error = null;
+        _exception = exception;
+    }
+
+    public T Value => _value ?? ThrowValueIsNotSetException();
+
+    public ApiError Error => _error ?? ThrowErrorIsNotSetException();
+
+    public Exception Exception => _exception ?? ThrowExceptionIsNotSetException();
+
+    public bool Success => _error == null && _exception == null;
+
+    public bool HasError => _error != null;
+
+    public bool HasException => _exception != null;
+
+    public static implicit operator NatsJSApiResult<T>(T value) => new(value);
+
+    public static implicit operator NatsJSApiResult<T>(ApiError error) => new(error);
+
+    public static implicit operator NatsJSApiResult<T>(Exception exception) => new(exception);
+
+    private static T ThrowValueIsNotSetException() => throw CreateInvalidOperationException("Result value is not set");
+
+    private static ApiError ThrowErrorIsNotSetException() => throw CreateInvalidOperationException("Result error is not set");
+
+    private static Exception ThrowExceptionIsNotSetException() => throw CreateInvalidOperationException("Result exception is not set");
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static Exception CreateInvalidOperationException(string message) => new InvalidOperationException(message);
+}

src/NATS.Client.JetStream/Internal/NatsJSJsonDocumentSerializer.cs

@@ -4,9 +4,38 @@
 
 namespace NATS.Client.JetStream.Internal;
 
-internal sealed class NatsJSJsonDocumentSerializer : INatsDeserialize<JsonDocument>
+internal sealed class NatsJSJsonDocumentSerializer<T> : INatsDeserialize<NatsJSApiResult<T>>
 {
-    public static readonly NatsJSJsonDocumentSerializer Default = new();
+    public static readonly NatsJSJsonDocumentSerializer<T> Default = new();
 
-    public JsonDocument? Deserialize(in ReadOnlySequence<byte> buffer) => buffer.Length == 0 ? default : JsonDocument.Parse(buffer);
+    public NatsJSApiResult<T> Deserialize(in ReadOnlySequence<byte> buffer)
+    {
+        if (buffer.Length == 0)
+        {
+            return new NatsJSException("Buffer is empty");
+        }
+
+        using var jsonDocument = JsonDocument.Parse(buffer);
+
+        if (jsonDocument.RootElement.TryGetProperty("error", out var errorElement))
+        {
+            var error = errorElement.Deserialize(JetStream.NatsJSJsonSerializerContext.Default.ApiError) ?? throw new NatsJSException("Can't parse JetStream error JSON payload");
+            return error;
+        }
+
+        var jsonTypeInfo = NatsJSJsonSerializerContext.DefaultContext.GetTypeInfo(typeof(T));
+        if (jsonTypeInfo == null)
+        {
+            return new NatsJSException($"Unknown response type {typeof(T)}");
+        }
+
+        var result = (T?)jsonDocument.RootElement.Deserialize(jsonTypeInfo);
+
+        if (result == null)
+        {
+            return new NatsJSException("Null result");
+        }
+
+        return result;
+    }
 }

src/NATS.Client.JetStream/NatsJSContext.cs

@@ -309,13 +309,13 @@ internal async ValueTask<NatsResult<NatsJSResponse<TResponse>>> TryJSRequestAsyn
             // Validator.ValidateObject(request, new ValidationContext(request));
         }
 
-        await using var sub = await Connection.CreateRequestSubAsync<TRequest, JsonDocument>(
+        await using var sub = await Connection.CreateRequestSubAsync<TRequest, NatsJSApiResult<TResponse>>(
                 subject: subject,
                 data: request,
                 headers: default,
                 replyOpts: new NatsSubOpts { Timeout = Connection.Opts.RequestTimeout },
                 requestSerializer: NatsJSJsonSerializer<TRequest>.Default,
-                replySerializer: NatsJSJsonDocumentSerializer.Default,
+                replySerializer: NatsJSJsonDocumentSerializer<TResponse>.Default,
                 cancellationToken: cancellationToken)
             .ConfigureAwait(false);
 
@@ -326,37 +326,22 @@ internal async ValueTask<NatsResult<NatsJSResponse<TResponse>>> TryJSRequestAsyn
                 return new NatsNoRespondersException();
             }
 
-            if (msg.Data == null)
-            {
-                return new NatsJSException("No response data received");
-            }
-
-            // We need to determine what type we're deserializing into
-            // .NET 6 new APIs to the rescue: we can read the buffer once
-            // by deserializing into a document, inspect and using the new
-            // API deserialize to the final type from the document.
-            using var jsonDocument = msg.Data;
-
-            if (jsonDocument.RootElement.TryGetProperty("error", out var errorElement))
+            if (msg.Error is { } messageError)
             {
-                var error = errorElement.Deserialize(JetStream.NatsJSJsonSerializerContext.Default.ApiError) ?? throw new NatsJSException("Can't parse JetStream error JSON payload");
-                return new NatsJSResponse<TResponse>(default, error);
+                return messageError;
             }
 
-            var jsonTypeInfo = NatsJSJsonSerializerContext.DefaultContext.GetTypeInfo(typeof(TResponse));
-            if (jsonTypeInfo == null)
+            if (msg.Data.HasException)
             {
-                return new NatsJSException($"Unknown response type {typeof(TResponse)}");
+                return msg.Data.Exception;
             }
 
-            var response = (TResponse?)jsonDocument.RootElement.Deserialize(jsonTypeInfo);
-
-            if (msg.Error is { } messageError)
+            if (msg.Data.HasError)
             {
-                return messageError;
+                return new NatsJSResponse<TResponse>(null, msg.Data.Error);
             }
 
-            return new NatsJSResponse<TResponse>(response, default);
+            return new NatsJSResponse<TResponse>(msg.Data.Value, null);
         }
 
         if (sub is NatsSubBase { EndReason: NatsSubEndReason.Exception, Exception: not null } sb)

tests/NATS.Client.JetStream.Tests/JetStreamApiSerializerTest.cs

@@ -1,4 +1,7 @@
+using System.Buffers;
+using System.Text;
 using NATS.Client.Core2.Tests;
+using NATS.Client.JetStream.Internal;
 using NATS.Client.JetStream.Models;
 using JsonSerializer = System.Text.Json.JsonSerializer;
 
@@ -98,4 +101,28 @@ public async Task Should_respect_buffers_lifecycle()
         {
         }
     }
+
+    [Fact]
+    public void Deserialize_value()
+    {
+        var serializer = NatsJSJsonDocumentSerializer<AccountInfoResponse>.Default;
+        var result = serializer.Deserialize(new ReadOnlySequence<byte>(Encoding.UTF8.GetBytes("""{"memory":1}""")));
+        result.Value.Memory.Should().Be(1);
+    }
+
+    [Fact]
+    public void Deserialize_empty_buffer()
+    {
+        var serializer = NatsJSJsonDocumentSerializer<AccountInfoResponse>.Default;
+        var result = serializer.Deserialize(ReadOnlySequence<byte>.Empty);
+        result.Exception.Message.Should().Be("Buffer is empty");
+    }
+
+    [Fact]
+    public void Deserialize_error()
+    {
+        var serializer = NatsJSJsonDocumentSerializer<AccountInfoResponse>.Default;
+        var result = serializer.Deserialize(new ReadOnlySequence<byte>(Encoding.UTF8.GetBytes("""{"error":{"code":2}}""")));
+        result.Error.Code.Should().Be(2);
+    }
 }