test(telegram): switch transient-flake retry test to group chat

Salvage follow-up. The transient thread-not-found retry test was
exercising chat_id='123' (positive, looks-like-private) which now
hits the new private-DM-topic fail-closed contract. The test's
intent is the transient-flake retry on real forum topics in groups,
so use -100123 to make the scenario unambiguous.

tests/gateway/test_telegram_thread_fallback.py

@@ -420,7 +420,7 @@ async def test_send_retries_transient_thread_not_found_before_fallback():
     adapter._bot = SimpleNamespace(send_message=mock_send_message)
 
     result = await adapter.send(
-        chat_id="123",
+        chat_id="-100123",
         content="test message",
         metadata={"thread_id": "99999"},
     )