fix(tui-gateway): count live rows in the 4033 archived_messages on a carrier cut

Gate C18 r1 quality [Low, unverified -> reproduced]: on the
durable-carrier path `truncated` is the physical durable_prefix, so
`len(history) - len(truncated)` mixed live and physical rows. A probe
with target row 101 absorbed into a repaired live carrier reported
archived_messages=5 for a cut that drops all 6 live rows (the carrier
and everything after it). The refusal now counts from the live cut
index on that path; the plain path is unchanged.

tests/tui_gateway/test_tui_gateway_server_deep_truncate.py

@@ -51,6 +51,17 @@ def test_prompt_submit_refuses_deep_truncation_without_confirm(monkeypatch):
 
         assert _submit(confirm_deep_truncate=True).get("error") is None
         assert replaced == [history[:2]]
+
+        # Durable-carrier cut: the target (101) was absorbed into a repaired live carrier;
+        # the refusal counts live rows from the carrier on, never live minus physical.
+        physical = [{"_row_id": 100, "role": "user", "content": "u0"},
+                    {"_row_id": 101, "role": "user", "content": "u0b"}, *history[1:]]
+        _FakeDB.get_messages_as_conversation = lambda self, key, **_kw: [dict(m) for m in physical]  # type: ignore[attr-defined]
+        sess["running"] = False  # the confirmed submit above left a (never-started) turn
+        sess["history"] = [{"_row_id": 100, "_absorbed_row_ids": [101], "role": "user",
+                            "content": "u0\n\nu0b"}, *history[1:]]
+        carrier = _submit(truncate_before_row_id=101)
+        assert carrier["error"]["data"] == {"archived_messages": 6, "archived_user_turns": 3}
     finally:
         server._sessions.pop("deep-trunc-sid", None)
 

tui_gateway/methods_prompt.py

@@ -466,7 +466,9 @@ def _truncate_history_for_submit(rid, sid, session, params, requested_rebind_ids
     # drop exactly one user turn, deeper cuts need confirm_deep_truncate.
     archived_user_turns = user_turn_count - ordinal
     if archived_user_turns > 1 and not is_truthy_value(params.get("confirm_deep_truncate")):
-        archived_messages = len(history) - len(truncated)
+        # A durable-carrier cut's ``truncated`` is physical rows: count the live cut instead.
+        archived_messages = len(history) - (
+            cut_index if durable_prefix is not None else len(truncated))
         logger.warning(
             "prompt.submit: REFUSED deep truncation of session %s (%d messages / %d user "
             "turns would be archived; ordinal=%d).",