A remote thread brings the replies that never reached us

The context of a remote post showed only what PrivaPub happened to hold:
replies from servers nobody here follows were never seen, and only the
ancestors were ever fetched. Now a persona opening a public remote thread
queues FetchReplies for the post and its root, at most once an hour each.

The job reads the thread's own collection first (FEP-7888 `context`, which
Mastodon 4.5+ serves with every reply at any depth; posts or, as FEP-f228
allows, the activities that made them), and otherwise the post's `replies`
(PeerTube's `comments`) and the replies' own, two levels down. At most 5
pages and 100 posts a job, signed by the instance actor, never a persona;
each post is fetched from its own origin and stored through StoreContext,
so only public and unlisted ones are kept.

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

PrivaPub.Tests/Http/MastodonStatusesTests.cs

@@ -1,5 +1,6 @@
 using MongoDB.Entities;
 
+using PrivaPub.Models.Jobs;
 using PrivaPub.Models.User;
 using PrivaPub.Tests.Support;
 using PrivaPub.Tests.Support.Host;
@@ -133,6 +134,36 @@ namespace PrivaPub.Tests.Http
 			Assert.Equal(1, (await alice.Client.Get($"/api/v1/statuses/{parent.ID}")).Ok().Body.Number("replies_count"));
 		}
 
+		// a remote thread opened by a persona has its replies read from its server (here its FEP-7888 context), at most hourly
+		[Fact]
+		public async Task Opening_a_remote_thread_reads_the_replies_that_never_reached_us()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var alice = await _host.Mastodon("alice");
+			var bob = new RemoteActor(_peer, "bob");
+			var carol = new RemoteActor(_peer, "carol");
+			var context = $"{bob.Origin()}/contexts/{Guid.NewGuid():N}";
+			var post = await _host.PublicPostFrom(bob, alice, shape: note => note["context"] = context);
+			var answer = carol.Create("<p>said elsewhere</p>", new[] { MastodonHelpers.Public }, new[] { carol.Id + "/followers" },
+				note => note["inReplyTo"] = post.ObjectURI)["object"]!;
+			var answerId = answer["id"]!.GetValue<string>();
+			_peer.Serve(new Uri(answerId).AbsolutePath, answer.ToJsonString());
+			_peer.Serve(new Uri(context).AbsolutePath, new JsonObject
+			{
+				["id"] = context, ["type"] = "OrderedCollection", ["orderedItems"] = new JsonArray(post.ObjectURI, answerId)
+			}.ToJsonString());
+			async Task<int> Queued() => (await DB.Default.Find<Job>().Match(j => j.Kind == JobKind.FetchReplies && j.Payload.Contains(post.ID)).ExecuteAsync(token)).Count;
+
+			(await _host.Client().Get($"/api/v1/statuses/{post.ID}/context")).Ok();
+			Assert.Equal(0, await Queued());
+			Assert.Empty((await alice.Client.Get($"/api/v1/statuses/{post.ID}/context")).Ok().Body["descendants"]!.AsArray());
+			Assert.Equal(1, await _host.Run(j => j.Kind == JobKind.FetchReplies && j.Payload.Contains(post.ID), token));
+
+			var descendants = (await alice.Client.Get($"/api/v1/statuses/{post.ID}/context")).Ok().Body["descendants"]!.AsArray();
+			Assert.Equal(answerId, Assert.Single(descendants).Text("uri"));
+			Assert.Equal(1, await Queued());
+		}
+
 		// Akkoma marks a post with sensitive media "sensitive" and leaves its summary empty: the media is hidden, the words are
 		// not, and no warning is made up for them (found by decePub's e2e tests on the town)
 		[Fact]