feat(matrix): reply in threads with fallback relations

Propagate Matrix thread metadata from inbound events and attach
m.relates_to
(rel_type=m.thread, m.in_reply_to, is_falling_back=true) to outbound
messages
including attachments. Add tests for thread metadata and thread replies.

tests/test_matrix_channel.py

@@ -510,6 +510,43 @@ async def test_on_message_room_mention_requires_opt_in() -> None:
     assert client.typing_calls == [("!room:matrix.org", True, TYPING_NOTICE_TIMEOUT_MS)]
 
 
+@pytest.mark.asyncio
+async def test_on_message_sets_thread_metadata_when_threaded_event() -> None:
+    channel = MatrixChannel(_make_config(), MessageBus())
+    client = _FakeAsyncClient("", "", "", None)
+    channel.client = client
+
+    handled: list[dict[str, object]] = []
+
+    async def _fake_handle_message(**kwargs) -> None:
+        handled.append(kwargs)
+
+    channel._handle_message = _fake_handle_message  # type: ignore[method-assign]
+
+    room = SimpleNamespace(room_id="!room:matrix.org", display_name="Test room", member_count=3)
+    event = SimpleNamespace(
+        sender="@alice:matrix.org",
+        body="Hello",
+        event_id="$reply1",
+        source={
+            "content": {
+                "m.relates_to": {
+                    "rel_type": "m.thread",
+                    "event_id": "$root1",
+                }
+            }
+        },
+    )
+
+    await channel._on_message(room, event)
+
+    assert len(handled) == 1
+    metadata = handled[0]["metadata"]
+    assert metadata["thread_root_event_id"] == "$root1"
+    assert metadata["thread_reply_to_event_id"] == "$reply1"
+    assert metadata["event_id"] == "$reply1"
+
+
 @pytest.mark.asyncio
 async def test_on_media_message_downloads_attachment_and_sets_metadata(
     monkeypatch, tmp_path
@@ -563,6 +600,51 @@ async def test_on_media_message_downloads_attachment_and_sets_metadata(
     assert "[attachment: " in handled[0]["content"]
 
 
+@pytest.mark.asyncio
+async def test_on_media_message_sets_thread_metadata_when_threaded_event(
+    monkeypatch, tmp_path
+) -> None:
+    monkeypatch.setattr("nanobot.channels.matrix.get_data_dir", lambda: tmp_path)
+
+    channel = MatrixChannel(_make_config(), MessageBus())
+    client = _FakeAsyncClient("", "", "", None)
+    client.download_bytes = b"image"
+    channel.client = client
+
+    handled: list[dict[str, object]] = []
+
+    async def _fake_handle_message(**kwargs) -> None:
+        handled.append(kwargs)
+
+    channel._handle_message = _fake_handle_message  # type: ignore[method-assign]
+
+    room = SimpleNamespace(room_id="!room:matrix.org", display_name="Test room", member_count=2)
+    event = SimpleNamespace(
+        sender="@alice:matrix.org",
+        body="photo.png",
+        url="mxc://example.org/mediaid",
+        event_id="$event1",
+        source={
+            "content": {
+                "msgtype": "m.image",
+                "info": {"mimetype": "image/png", "size": 5},
+                "m.relates_to": {
+                    "rel_type": "m.thread",
+                    "event_id": "$root1",
+                },
+            }
+        },
+    )
+
+    await channel._on_media_message(room, event)
+
+    assert len(handled) == 1
+    metadata = handled[0]["metadata"]
+    assert metadata["thread_root_event_id"] == "$root1"
+    assert metadata["thread_reply_to_event_id"] == "$event1"
+    assert metadata["event_id"] == "$event1"
+
+
 @pytest.mark.asyncio
 async def test_on_media_message_respects_declared_size_limit(
     monkeypatch, tmp_path
@@ -801,6 +883,34 @@ async def test_send_uploads_media_and_sends_file_event(tmp_path) -> None:
     assert client.room_send_calls[1]["content"]["body"] == "Please review."
 
 
+@pytest.mark.asyncio
+async def test_send_adds_thread_relates_to_for_thread_metadata() -> None:
+    channel = MatrixChannel(_make_config(), MessageBus())
+    client = _FakeAsyncClient("", "", "", None)
+    channel.client = client
+
+    metadata = {
+        "thread_root_event_id": "$root1",
+        "thread_reply_to_event_id": "$reply1",
+    }
+    await channel.send(
+        OutboundMessage(
+            channel="matrix",
+            chat_id="!room:matrix.org",
+            content="Hi",
+            metadata=metadata,
+        )
+    )
+
+    content = client.room_send_calls[0]["content"]
+    assert content["m.relates_to"] == {
+        "rel_type": "m.thread",
+        "event_id": "$root1",
+        "m.in_reply_to": {"event_id": "$reply1"},
+        "is_falling_back": True,
+    }
+
+
 @pytest.mark.asyncio
 async def test_send_uses_encrypted_media_payload_in_encrypted_room(tmp_path) -> None:
     channel = MatrixChannel(_make_config(e2ee_enabled=True), MessageBus())
@@ -851,6 +961,50 @@ async def test_send_does_not_parse_attachment_marker_without_media(tmp_path) ->
     assert client.room_send_calls[0]["content"]["body"] == f"[attachment: {missing_path}]"
 
 
+@pytest.mark.asyncio
+async def test_send_passes_thread_relates_to_to_attachment_upload(monkeypatch) -> None:
+    channel = MatrixChannel(_make_config(), MessageBus())
+    client = _FakeAsyncClient("", "", "", None)
+    channel.client = client
+    channel._server_upload_limit_checked = True
+    channel._server_upload_limit_bytes = None
+
+    captured: dict[str, object] = {}
+
+    async def _fake_upload_and_send_attachment(
+        *,
+        room_id: str,
+        path: Path,
+        limit_bytes: int,
+        relates_to: dict[str, object] | None = None,
+    ) -> str | None:
+        captured["relates_to"] = relates_to
+        return None
+
+    monkeypatch.setattr(channel, "_upload_and_send_attachment", _fake_upload_and_send_attachment)
+
+    metadata = {
+        "thread_root_event_id": "$root1",
+        "thread_reply_to_event_id": "$reply1",
+    }
+    await channel.send(
+        OutboundMessage(
+            channel="matrix",
+            chat_id="!room:matrix.org",
+            content="Hi",
+            media=["/tmp/fake.txt"],
+            metadata=metadata,
+        )
+    )
+
+    assert captured["relates_to"] == {
+        "rel_type": "m.thread",
+        "event_id": "$root1",
+        "m.in_reply_to": {"event_id": "$reply1"},
+        "is_falling_back": True,
+    }
+
+
 @pytest.mark.asyncio
 async def test_send_workspace_restriction_blocks_external_attachment(tmp_path) -> None:
     workspace = tmp_path / "workspace"