feat(webui): add assistant reply fork-from-here

tests/utils/test_webui_transcript.py

@@ -6,8 +6,10 @@ from nanobot.webui.transcript import (
     WEBUI_TRANSCRIPT_SCHEMA_VERSION,
     append_transcript_object,
     build_webui_thread_response,
+    fork_transcript_before_user_index,
     read_transcript_lines,
     replay_transcript_to_ui_messages,
+    write_session_messages_as_transcript,
 )
 
 
@@ -20,6 +22,79 @@ def test_append_and_read_roundtrip(tmp_path, monkeypatch) -> None:
     assert lines[0]["text"] == "hello"
 
 
+def test_fork_transcript_before_user_index_copies_only_prefix(tmp_path, monkeypatch) -> None:
+    monkeypatch.setattr("nanobot.config.paths.get_data_dir", lambda: tmp_path)
+    source = "websocket:source"
+    for ev in (
+        {"event": "user", "chat_id": "source", "text": "round1"},
+        {"event": "message", "chat_id": "source", "text": "answer1"},
+        {"event": "turn_end", "chat_id": "source"},
+        {"event": "user", "chat_id": "source", "text": "round2 fork me"},
+        {"event": "message", "chat_id": "source", "text": "answer2"},
+        {"event": "user", "chat_id": "source", "text": "round3 must not appear"},
+    ):
+        append_transcript_object(source, ev)
+
+    ok = fork_transcript_before_user_index(source, "websocket:fork", 1)
+
+    assert ok is True
+    lines = read_transcript_lines("websocket:fork")
+    assert [line.get("text") for line in lines] == ["round1", "answer1", None]
+    assert all(line.get("chat_id") == "fork" for line in lines)
+    assert "round2 fork me" not in "\n".join(str(line.get("text")) for line in lines)
+    assert "round3 must not appear" not in "\n".join(str(line.get("text")) for line in lines)
+
+
+def test_fork_transcript_rejects_out_of_range_user_index(tmp_path, monkeypatch) -> None:
+    monkeypatch.setattr("nanobot.config.paths.get_data_dir", lambda: tmp_path)
+    source = "websocket:source"
+    append_transcript_object(source, {"event": "user", "chat_id": "source", "text": "round1"})
+
+    assert fork_transcript_before_user_index(source, "websocket:fork", 2) is False
+    assert read_transcript_lines("websocket:fork") == []
+
+
+def test_fork_transcript_allows_index_equal_to_user_count(tmp_path, monkeypatch) -> None:
+    monkeypatch.setattr("nanobot.config.paths.get_data_dir", lambda: tmp_path)
+    source = "websocket:source"
+    for ev in (
+        {"event": "user", "chat_id": "source", "text": "round1"},
+        {"event": "message", "chat_id": "source", "text": "answer1"},
+    ):
+        append_transcript_object(source, ev)
+
+    ok = fork_transcript_before_user_index(source, "websocket:fork", 1)
+
+    assert ok is True
+    assert [line.get("text") for line in read_transcript_lines("websocket:fork")] == [
+        "round1",
+        "answer1",
+    ]
+
+
+def test_write_session_messages_as_transcript_builds_canonical_prefix(
+    tmp_path,
+    monkeypatch,
+) -> None:
+    monkeypatch.setattr("nanobot.config.paths.get_data_dir", lambda: tmp_path)
+
+    write_session_messages_as_transcript(
+        "websocket:fork",
+        [
+            {"role": "user", "content": "round1"},
+            {"role": "assistant", "content": "answer1"},
+        ],
+    )
+
+    lines = read_transcript_lines("websocket:fork")
+    assert lines == [
+        {"event": "user", "chat_id": "fork", "text": "round1"},
+        {"event": "message", "chat_id": "fork", "text": "answer1"},
+    ]
+    msgs = replay_transcript_to_ui_messages(lines)
+    assert [m["content"] for m in msgs] == ["round1", "answer1"]
+
+
 def test_replay_delta_and_turn_end(tmp_path, monkeypatch) -> None:
     monkeypatch.setattr("nanobot.config.paths.get_data_dir", lambda: tmp_path)
     key = "websocket:t2"