Add Polly resilience policies

clients/dotnet/MxGateway.Client.Tests/FakeGatewayTransport.cs

@@ -36,11 +36,22 @@ internal sealed class FakeGatewayTransport(MxGatewayClientOptions options) : IMx
         ProtocolStatus = new ProtocolStatus { Code = ProtocolStatusCode.Ok },
     };
 
+    public Queue<Exception> OpenSessionExceptions { get; } = new();
+
+    public Queue<Exception> CloseSessionExceptions { get; } = new();
+
+    public Queue<Exception> InvokeExceptions { get; } = new();
+
     public Task<OpenSessionReply> OpenSessionAsync(
         OpenSessionRequest request,
         CallOptions callOptions)
     {
         OpenSessionCalls.Add((request, callOptions));
+        if (OpenSessionExceptions.TryDequeue(out Exception? exception))
+        {
+            throw exception;
+        }
+
         return Task.FromResult(OpenSessionReply);
     }
 
@@ -49,6 +60,11 @@ internal sealed class FakeGatewayTransport(MxGatewayClientOptions options) : IMx
         CallOptions callOptions)
     {
         CloseSessionCalls.Add((request, callOptions));
+        if (CloseSessionExceptions.TryDequeue(out Exception? exception))
+        {
+            throw exception;
+        }
+
         return Task.FromResult(CloseSessionReply);
     }
 
@@ -57,6 +73,11 @@ internal sealed class FakeGatewayTransport(MxGatewayClientOptions options) : IMx
         CallOptions callOptions)
     {
         InvokeCalls.Add((request, callOptions));
+        if (InvokeExceptions.TryDequeue(out Exception? exception))
+        {
+            throw exception;
+        }
+
         return Task.FromResult(_invokeReplies.Dequeue());
     }
 

clients/dotnet/MxGateway.Client.Tests/MxGatewayClientOptionsTests.cs

@@ -25,4 +25,17 @@ public sealed class MxGatewayClientOptionsTests
 
         Assert.Throws<ArgumentException>(options.Validate);
     }
+
+    [Fact]
+    public void Validate_WithInvalidRetryOptions_Throws()
+    {
+        var options = new MxGatewayClientOptions
+        {
+            Endpoint = new Uri("http://localhost:5000"),
+            ApiKey = "test-api-key",
+            Retry = new MxGatewayClientRetryOptions { MaxAttempts = 0 },
+        };
+
+        Assert.Throws<ArgumentOutOfRangeException>(options.Validate);
+    }
 }

clients/dotnet/MxGateway.Client.Tests/MxGatewayClientSessionTests.cs

@@ -1,4 +1,5 @@
 using MxGateway.Contracts.Proto;
+using Grpc.Core;
 
 namespace MxGateway.Client.Tests;
 
@@ -220,6 +221,55 @@ public sealed class MxGatewayClientSessionTests
         Assert.Equal("session-fixture", call.Request.SessionId);
     }
 
+    [Fact]
+    public async Task InvokeAsync_RetriesSafeDiagnosticCommandOnTransientGrpcFailure()
+    {
+        FakeGatewayTransport transport = CreateTransport();
+        transport.InvokeExceptions.Enqueue(CreateTransientRpcException());
+        transport.AddInvokeReply(new MxCommandReply
+        {
+            SessionId = "session-fixture",
+            Kind = MxCommandKind.Ping,
+            ProtocolStatus = new ProtocolStatus { Code = ProtocolStatusCode.Ok },
+        });
+        await using MxGatewayClient client = CreateClient(transport);
+        MxGatewaySession session = await client.OpenSessionAsync();
+
+        await session.InvokeAsync(new MxCommandRequest
+        {
+            SessionId = session.SessionId,
+            Command = new MxCommand { Kind = MxCommandKind.Ping, Ping = new PingCommand() },
+        });
+
+        Assert.Equal(2, transport.InvokeCalls.Count);
+    }
+
+    [Fact]
+    public async Task OpenSessionAsync_DoesNotRetryTransientGrpcFailure()
+    {
+        FakeGatewayTransport transport = CreateTransport();
+        transport.OpenSessionExceptions.Enqueue(CreateTransientRpcException());
+        await using MxGatewayClient client = CreateClient(transport);
+
+        await Assert.ThrowsAsync<RpcException>(async () => await client.OpenSessionAsync());
+
+        Assert.Single(transport.OpenSessionCalls);
+    }
+
+    [Fact]
+    public async Task InvokeAsync_DoesNotRetryWriteCommand()
+    {
+        FakeGatewayTransport transport = CreateTransport();
+        transport.InvokeExceptions.Enqueue(CreateTransientRpcException());
+        await using MxGatewayClient client = CreateClient(transport);
+        MxGatewaySession session = await client.OpenSessionAsync();
+
+        await Assert.ThrowsAsync<RpcException>(async () =>
+            await session.WriteRawAsync(1, 2, 3.ToMxValue(), userId: 0));
+
+        Assert.Single(transport.InvokeCalls);
+    }
+
     [Fact]
     public async Task InvokeHelpers_PassCancellationTokenToTransport()
     {
@@ -252,4 +302,9 @@ public sealed class MxGatewayClientSessionTests
             ApiKey = "test-api-key",
         });
     }
+
+    private static RpcException CreateTransientRpcException()
+    {
+        return new RpcException(new Status(StatusCode.Unavailable, "gateway unavailable"));
+    }
 }