Fixes blockquote > escaping, edit_message raw markdown, ***bold italic***
handling, HTML entity double-escaping (&), Wikipedia URL parens
truncation, and step numbering format. Also adds format_message to the
tool-layer _send_to_platform for consistent formatting across all
delivery paths.
Changes:
- Protect Slack entities (<@user>, <https://...|label>, <!here>) from
escaping passes
- Protect blockquote > markers before HTML entity escaping
- Unescape-before-escape for idempotent HTML entity handling
- ***bold italic*** → *_text_* conversion (before **bold** pass)
- URL regex upgraded to handle balanced parentheses
- mrkdwn:True flag on chat_postMessage payloads
- format_message applied in edit_message and send_message_tool
- 52 new tests (format, edit, streaming, splitting, tool chunking)
- Use reversed(dict) idiom for placeholder restoration
Based on PR #3715 by dashed, cherry-picked onto current main.
