fix(gateway): send markdown media as documents

[lukas-runge-bot]

Summary

• recognize unquoted .md and .markdown MEDIA paths in gateway extraction
• cover markdown MEDIA extraction with regression tests
• ensure standalone Telegram delivery sends markdown attachments via send_document instead of plain text

Test plan

• venv/bin/python -m pytest tests/gateway/test_platform_base.py::TestExtractMedia tests/tools/test_send_message_telegram_proxy.py -q -o 'addopts='

[alt-glitch]

Competing with #29374, #31561, #33269, #33206, #34724 and others for the same MEDIA markdown attachment fix. Root issue: #31560. Note: #29609 (dynamic derivation) is the preferred approach per prior triage.