fix: merge-last-assistant deletes tool/system rows from the DB (history desync) (#1929)

tests/test_merge_last_assistant_rows.py

@@ -0,0 +1,41 @@
+"""merge-last-assistant must not delete tool/system rows between the messages.
+
+The in-memory merge removes the second assistant message plus only the
+"continue" user message between the last two assistant replies. The DB path
+deleted the ENTIRE index range between them, destroying any tool/system/user
+rows in between — so on reload the DB lost messages the in-memory history
+kept (data loss + count desync). _merge_continue_rows_to_delete makes the DB
+deletion mirror the in-memory rule.
+"""
+from types import SimpleNamespace
+
+from routes.history_routes import _merge_continue_rows_to_delete
+
+
+def _m(role, content=""):
+    return SimpleNamespace(role=role, content=content)
+
+
+def test_tool_message_between_is_not_deleted():
+    u, a1, tool, a2 = _m("user", "q"), _m("assistant", "a1"), _m("tool", "RESULT"), _m("assistant", "a2")
+    rows = _merge_continue_rows_to_delete([u, a1, tool, a2], a1, a2)
+    assert rows == [a2]            # only the 2nd assistant
+    assert tool not in rows        # the tool result survives
+
+
+def test_continue_user_message_is_deleted():
+    u, a1, cont, a2 = (_m("user", "q"), _m("assistant", "a1"),
+                       _m("user", "(the previous response was interrupted)"), _m("assistant", "a2"))
+    rows = _merge_continue_rows_to_delete([u, a1, cont, a2], a1, a2)
+    assert a2 in rows and cont in rows and len(rows) == 2
+
+
+def test_adjacent_assistants_delete_only_second():
+    a1, a2 = _m("assistant", "a1"), _m("assistant", "a2")
+    assert _merge_continue_rows_to_delete([a1, a2], a1, a2) == [a2]
+
+
+def test_plain_user_between_not_deleted():
+    a1, usr, a2 = _m("assistant", "a1"), _m("user", "a real follow-up question"), _m("assistant", "a2")
+    rows = _merge_continue_rows_to_delete([a1, usr, a2], a1, a2)
+    assert rows == [a2] and usr not in rows