Server: start a retried write transaction from the database, not from the attempt that failed

tests/DodoSSH.Api.Tests/ApiFixture.cs

@@ -14,6 +14,7 @@ using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Testcontainers.PostgreSql;
 using Xunit;
+using Microsoft.EntityFrameworkCore.Diagnostics;
 
 namespace DodoSSH.Api.Tests;
 
@@ -108,9 +109,16 @@ public sealed class ApiFixture : WebApplicationFactory<Program>, IAsyncLifetime
             Environment.SetEnvironmentVariable(name, value);
         }
 
-        builder.ConfigureTestServices(services => services.AddSingleton<IRelayNetwork>(RelayNetwork));
+        builder.ConfigureTestServices(services =>
+        {
+            services.AddSingleton<IRelayNetwork>(RelayNetwork);
+            services.AddSingleton<IInterceptor>(Commits);
+        });
     }
 
+    /// <summary>Fails a commit on request, so a test can watch the retrying execution strategy at work.</summary>
+    internal FlakyCommits Commits { get; } = new();
+
     /// <summary>
     /// What the relay resolves and dials, in place of the machine's own resolver and sockets.
     /// </summary>