Mongo is a one-member replica set

Owner decision 2026-10-07: production's mongod becomes a one-member replica set (rs0), so that a backup can read every
collection at one instant (a snapshot read session), which a standalone mongod can't. The unit runs mongod with
--replSet rs0 and a 990 MB oplog; setup.sh converts it once, idempotently (PrivaPub stopped, mongod restarted,
rs.initiate, the primary awaited, PrivaPub started); every connection string says directConnection=true, which works
against the standalone too, so this code can deploy before the conversion. The CI's throwaway mongod and the pasture's
are replica sets as well, so the snapshot path is what the tests exercise; MongoTopology tells which a mongod is, and
TopologyTests holds the test mongod to it. The suite passes on it (906).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

PrivaPub.Tests/Infrastructure/TopologyTests.cs

@@ -0,0 +1,21 @@
+using MongoDB.Entities;
+
+using PrivaPub.Infrastructure.Data;
+using PrivaPub.Tests.Support;
+
+namespace PrivaPub.Tests.Infrastructure
+{
+	[Trait("Category", "Integration")]
+	public sealed class TopologyTests
+	{
+		// tools/ci/with-test-mongod.sh starts a one-member replica set, as production's mongod is: the backup's snapshot reads
+		// are what the tests exercise
+		[Fact]
+		public async Task The_test_mongod_is_a_replica_set_like_productions()
+		{
+			Assert.SkipUnless(MongoFixture.Enabled, MongoFixture.Skip);
+			Assert.SkipUnless(MongoFixture.Connection.Contains("directConnection=true"), "a mongod not started by tools/ci/with-test-mongod.sh");
+			Assert.True(await MongoTopology.IsReplicaSet(DB.Default.Database(), TestContext.Current.CancellationToken));
+		}
+	}
+}