feat: render markdown tables for Slack

[noamoc]

Summary

• Detect GFM pipe tables in Slack outbound formatting
• Render tables as aligned monospaced code blocks so numeric reports stay readable
• Leave fenced code blocks and prose with literal pipes untouched

Test Plan

• python -m pytest tests/gateway/test_slack_table_format.py -q -o 'addopts='

[alt-glitch]

Duplicate of #15854 (same approach: GFM pipe tables → monospace code blocks for Slack). Also superseded by #16648 which adds CJK-aware alignment. Addresses #18918.