A deleted post's id given to a new post is that new post

PrivaPub remembers a deleted remote post's id for 90 days so that a late Create cannot bring it back. Gancio numbers a
new event after the last one it keeps, so deleting its last event gives the next the same id, and that event was
dropped as deleted. A Create published after the deletion is now kept as the new post; the deleted one's own Create,
however late, still is not. Checked live: Gancio's scenario passes again (17 checks).

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

PrivaPub.Tests/Federation/ReusedIdTests.cs

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+using MongoDB.Entities;
+
+using PrivaPub.Models.Federation;
+using PrivaPub.Models.Post;
+using PrivaPub.Models.Social;
+using PrivaPub.Tests.Support;
+
+using System.Text.Json.Nodes;
+
+using static PrivaPub.Tests.Support.FederatedSeeds;
+
+namespace PrivaPub.Tests.Federation
+{
+	// A deleted remote post is never brought back by its own Create, however late it comes; an id its server gives a new
+	// post afterwards (Gancio numbers an event after the last one it keeps) is that new post
+	[Trait("Category", "Integration")]
+	public sealed class ReusedIdTests : IAsyncLifetime
+	{
+		Harness _harness;
+
+		public async ValueTask InitializeAsync()
+		{
+			Assert.SkipUnless(MongoFixture.Enabled, MongoFixture.Skip);
+			_harness = await Harness.Start();
+		}
+
+		public async ValueTask DisposeAsync()
+		{
+			if (_harness != default)
+				await _harness.DisposeAsync();
+		}
+
+		[Fact]
+		public async Task A_deleted_posts_create_stays_dropped_and_a_new_post_under_its_id_is_kept()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var (_, alice) = await _harness.Persona("alice");
+			var carol = new RemoteActor(_harness.Peer, "carol");
+			await Follows(alice.Id, carol);
+			var first = PublicNote(carol, "<p>the first event</p>");
+			first["published"] = DateTime.UtcNow.AddMinutes(-10).ToString("O");
+			await _harness.Deliver(carol, "/human-centipede", Create(carol, first));
+			await _harness.Deliver(carol, "/human-centipede", new JsonObject
+			{
+				["id"] = NewId(carol, "deletes"), ["type"] = "Delete", ["actor"] = carol.Id,
+				["object"] = new JsonObject { ["id"] = IdOf(first), ["type"] = "Tombstone" }
+			});
+			Assert.True(await DB.Default.Find<DeletedObject>().Match(d => d.ObjectURI == IdOf(first)).ExecuteAnyAsync(token));
+
+			// the first Create again, late
+			await _harness.Deliver(carol, "/human-centipede", Create(carol, (JsonObject)first.DeepClone()));
+			Assert.False(await DB.Default.Find<Post>().Match(p => p.ObjectURI == IdOf(first) && !p.DeletedAt.HasValue).ExecuteAnyAsync(token));
+
+			var second = PublicNote(carol, "<p>a new event, numbered as the first</p>");
+			second["id"] = IdOf(first);
+			second["published"] = DateTime.UtcNow.ToString("O");
+			await _harness.Deliver(carol, "/human-centipede", Create(carol, second));
+
+			var kept = await DB.Default.Find<Post>().Match(p => p.ObjectURI == IdOf(first) && !p.DeletedAt.HasValue).ExecuteFirstAsync(token);
+			Assert.Contains("a new event", kept?.ContentHtml);
+			Assert.False(await DB.Default.Find<DeletedObject>().Match(d => d.ObjectURI == IdOf(first)).ExecuteAnyAsync(token));
+		}
+	}
+}