Preserve rich formatting on message edits

[delaneyb]

Summary

Preserves supported rich formatting when editing Slack messages via chat.update, without changing plain text edit behavior.

What changed

• Adds an opt-in mode to the existing rich-text block converter for inline formatting such as links, bold, italic, strike, inline code, mentions, and broadcasts.
• Uses that mode from message edit so formatted edits include Slack rich_text blocks.
• Keeps plain text edits text-only.
• Keeps existing message send behavior unchanged.

Tests

• Plain text edits do not send blocks.
• Literal angle-bracket text such as <fix> stays escaped text and does not become blocks.
• Message URL edits with <https://example.com|label> send rich-text blocks.
• Channel + timestamp edits with formatting send rich-text blocks.
• Rich-text converter default behavior is unchanged unless the new opt-in flag is used.

Prepared by GPT-5.5 Medium using a custom sub-agent review process.

This change was reviewed by 5 sub-agents:

• 1 decomposition reviewer.
• 1 bailout-gate reviewer for the chat.update formatting approach.
• 2 implementation/test/maintainer reviewers.
• 1 follow-up confirmation reviewer for the literal angle-bracket regression fix.