fix(telegram): address code review issues from cherry-pick merge

- Fix critical plain-text fallback that was sending raw HTML tags to
  users: keep raw markdown available for the fallback path
- Extract TELEGRAM_HTML_MAX_LEN (4096) constant to replace hardcoded
  magic number and document the difference from TELEGRAM_MAX_MESSAGE_LEN
- Add fallback to _send_text for extra HTML chunks when HTML parse fails
- Add missing @pytest.mark.asyncio decorator on
  test_send_delta_stream_end_html_expansion_does_not_overflow

tests/channels/test_telegram_channel.py

@@ -497,6 +497,7 @@ async def test_send_delta_stream_end_splits_oversized_reply() -> None:
     assert "123" not in channel._stream_bufs
 
 
+@pytest.mark.asyncio
 async def test_send_delta_stream_end_html_expansion_does_not_overflow() -> None:
     """Markdown that expands when converted to HTML is still split correctly.