chore: clarify regenerate lifecycle warning wording (T90.2)

The warning said "lifecycle transitions from superseded turn ARE NOT
being rolled back". When regenerating an OLDER turn, the listed
transitions can include intervening-turn ones that legitimately stand
on their own — they weren't authored by the superseded turn itself.

Reword to "lifecycle transitions at-or-after turn <id>" so operators
reading logs aren't misled into thinking every listed event id was
emitted by the target turn. Cosmetic change to a single log message.

Test: extends test_regenerate_with_prior_lifecycle_logs_warning to
assert the new phrasing is present and the old phrasing is gone.

chat/services/regenerate.py

@@ -182,9 +182,13 @@ async def regenerate_assistant_turn(
         (chat_id, original_assistant_event_id),
     ).fetchall()
     if unrolled_lifecycle:
+        # T90.2: phrased as "at-or-after turn <id>" rather than "from
+        # superseded turn" because regenerating an OLDER turn lists
+        # intervening-turn transitions that legitimately stand on their
+        # own — those weren't authored by the superseded turn itself.
         _log.warning(
-            "regenerate_assistant_turn: %d lifecycle transition(s) from "
-            "superseded turn %s are NOT being rolled back (Phase 4 "
+            "regenerate_assistant_turn: %d lifecycle transition(s) "
+            "at-or-after turn %s are NOT being rolled back (Phase 4 "
             "follow-up). Affected event ids: %s",
             len(unrolled_lifecycle),
             original_assistant_event_id,