chore: support setting an execution time for Commit in .NET mock server (#568)

Adding an ExecutionTime (for example an error) had no effect for the Commit method
in the .NET mock server.

Author: olavloite

spannerlib/wrappers/spannerlib-dotnet/spannerlib-dotnet-mockserver/MockSpannerServer.cs

@@ -420,6 +420,8 @@ public override Task<CommitResponse> Commit(CommitRequest request, ServerCallCon
         _contexts.Enqueue(context);
         _headers.Enqueue(context.RequestHeaders);
         TryFindSession(request.SessionAsSessionName);
+        _executionTimes.TryGetValue(nameof(Commit), out var executionTime);
+        executionTime?.SimulateExecutionTime();
         if (request.TransactionCase == CommitRequest.TransactionOneofCase.TransactionId)
         {
             TryFindTransaction(request.TransactionId, true);

spannerlib/wrappers/spannerlib-dotnet/spannerlib-dotnet-tests/TransactionTests.cs

@@ -16,6 +16,7 @@
 using Google.Cloud.SpannerLib.MockServer;
 using Google.Protobuf.WellKnownTypes;
 using Google.Rpc;
+using Grpc.Core;
 
 namespace Google.Cloud.SpannerLib.Tests;
 
@@ -133,4 +134,16 @@ public void TestBeginTwice([Values] LibType libType)
         var exception = Assert.Throws<SpannerException>(() => connection.BeginTransaction(new TransactionOptions()));
         Assert.That(exception.Code, Is.EqualTo(Code.FailedPrecondition));
     }
+
+    [Test]
+    public void TestCommitFailed([Values] LibType libType)
+    {
+        Fixture.SpannerMock.AddOrUpdateExecutionTime(nameof(Fixture.SpannerMock.Commit), ExecutionTime.CreateException(StatusCode.FailedPrecondition, "Invalid mutations"));
+        
+        using var pool = Pool.Create(SpannerLibDictionary[libType], ConnectionString);
+        using var connection = pool.CreateConnection();
+        var exception = Assert.Throws<SpannerException>(() => connection.WriteMutations(new BatchWriteRequest.Types.MutationGroup()));
+        Assert.That(exception.Code, Is.EqualTo(Code.FailedPrecondition));
+    }
+    
 }