P7: thread backfill reads conversation containers and asks with If-None-Match

The FetchReplies job reads a thread's FEP-171b contextHistory before its FEP-7888 context: the posts each Add (or
Forte's plain Create) brought in, read from their own servers. A thread collection read whole keeps its ETag when the
document itself changes with every post (it counts them, or holds them all with no further page); the next read sends
it as If-None-Match and a 304 ends the job. The ETag is kept as sent, since NodeBB's has no quotes and the typed header
drops it. A context naming a post we hold (Forte's first post) is not fetched.

Checked in the pasture: Mastodon 4.7.3 answers the second read 304; NodeBB 4.16's unquoted ETag is kept (23/23 in its
scenario); a Forte thread completes through its replies.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub.Tests/Support/Peer.cs

@@ -21,6 +21,7 @@ namespace PrivaPub.Tests.Support
 		readonly ConcurrentDictionary<string, (string Text, string ContentType)> _documents = new();
 		readonly ConcurrentDictionary<string, (int Status, TimeSpan Delay)> _answers = new();
 		readonly ConcurrentDictionary<string, (byte[] Bytes, string ContentType)> _files = new();
+		readonly ConcurrentDictionary<string, string> _etags = new();
 
 		public int Port { get; }
 		public string A => $"http://127.0.0.1:{Port}";
@@ -64,6 +65,15 @@ namespace PrivaPub.Tests.Support
 					context.Response.StatusCode = StatusCodes.Status404NotFound;
 					return;
 				}
+				if (peer._etags.TryGetValue(key, out var etag))
+				{
+					if (context.Request.Headers.IfNoneMatch.ToString() == etag)
+					{
+						context.Response.StatusCode = StatusCodes.Status304NotModified;
+						return;
+					}
+					context.Response.Headers.ETag = etag;
+				}
 				context.Response.ContentType = document.ContentType;
 				await context.Response.WriteAsync(document.Text.Replace("{A}", peer.A).Replace("{B}", peer.B));
 			});
@@ -74,6 +84,13 @@ namespace PrivaPub.Tests.Support
 
 		public void Serve(string path, string json) => _documents[path] = (json, "application/activity+json");
 
+		// a document with an ETag: answered 304 to a request that names it in If-None-Match
+		public void ServeTagged(string path, string json, string etag)
+		{
+			_documents[path] = (json, "application/activity+json");
+			_etags[path] = etag;
+		}
+
 		public void ServeText(string path, string text, string contentType) => _documents[path] = (text, contentType);
 
 		public void ServeFile(string path, byte[] bytes, string contentType) => _files[path] = (bytes, contentType);