Building on PR #288's edit_message() abstraction:
- Telegram: edit_message_text() with MarkdownV2 + plain text fallback
- Discord: channel.fetch_message() + msg.edit() with length capping
- Slack: chat_update() via slack_bolt client
Also fixes the fallback regression in send_progress_messages() where
platforms that don't support editing would receive duplicated accumulated
tool lines. Now uses a can_edit flag — after the first failed edit, falls
back to sending individual lines (matching pre-PR behavior).
1708dcd2b2