fix: avoid replaying older long turns

Treat the current live user message as the replay boundary for normal user turns, while keeping user-turn extension for history and consolidation paths that need it. Add regression coverage for the user-triggered long tool-turn case.

tests/agent/test_session_manager_history.py

@@ -660,6 +660,23 @@ def test_get_history_can_extend_to_user_for_long_recent_turn():
     _assert_no_orphans(extended)
 
 
+def test_get_history_extend_to_user_keeps_newer_user_inside_window():
+    session = Session(key="test:history-extend-newer-user")
+    session.messages.append({"role": "user", "content": "old"})
+    session.messages.append({"role": "assistant", "content": "old answer"})
+    session.messages.append({"role": "user", "content": "long older turn"})
+    for i in range(8):
+        session.messages.extend(_tool_turn("older", i))
+    session.messages.append({"role": "assistant", "content": "older final"})
+    session.messages.append({"role": "user", "content": "new question"})
+    session.messages.append({"role": "assistant", "content": "new answer"})
+
+    history = session.get_history(max_messages=6, extend_to_user=True)
+
+    assert [m["content"] for m in history] == ["new question", "new answer"]
+    _assert_no_orphans(history)
+
+
 # --- enforce_file_cap archive correctness (issue #4128) ---