Matrix threads in the transport (M2.1, PR A)

Transport.post(text, thread=...) returns the event ID, which can root a
thread; a threaded post carries m.thread with the reply fallback for
clients without threads. Inbound messages carry their event ID and thread
root: a message in a thread gives its root, a plain reply joins the
thread of the message it replies to when known, and the quoted lines of a
rich reply are no longer part of the text. The in-memory transport plays
threads for tests.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

tests/integration/test_matrix_live.py

@@ -51,3 +51,25 @@ def test_send_lands_in_test_room(live_config: AgentConfig) -> None:
         assert marker in bodies
     finally:
         transport.close()
+
+
+def test_a_threaded_post_is_kept_in_its_thread(live_config: AgentConfig) -> None:
+    """The homeserver accepts the thread relation and lists the reply under
+    its root (L5: a work item's conversation is a thread)."""
+    config = live_config.matrix
+    marker = uuid.uuid4().hex
+    transport = MatrixTransport(config)
+    try:
+        _assert_testing_identity(transport, config)
+        root = transport.post(f"thread smoke test root {marker}")
+        reply = transport.post(f"thread smoke test reply {marker}", thread=root)
+        # The relations API is v1, not the transport's v3.
+        response = transport._http.get(
+            f"{config.homeserver}/_matrix/client/v1/rooms/{config.room_id}"
+            f"/relations/{root}/m.thread"
+        )
+        response.raise_for_status()
+        in_thread = [e["event_id"] for e in response.json().get("chunk", [])]
+        assert reply in in_thread
+    finally:
+        transport.close()