fix: keep Telegram chunk markers outside code fences

[whoislikemiha]

Summary

• move Telegram chunk indicators off closing fenced-code lines before sending MarkdownV2
• apply the same repair to legacy send chunks and overflow finalization chunks
• add a regression test for long streamed/legacy Telegram replies with fenced code blocks

Test Plan

• uv run --with pytest --with pytest-asyncio python -m pytest tests/gateway/test_telegram_format.py::test_legacy_send_keeps_chunk_indicators_outside_fenced_code_lines -q
• uv run --with pytest --with pytest-asyncio python -m pytest tests/gateway/test_telegram_format.py tests/gateway/test_telegram_overflow_partial.py tests/gateway/test_telegram_rich_messages.py -q