Keep Slack previews with custom outbound identity

Slack users with custom outbound identity could lose portable draft-preview streaming when native streaming was unavailable. Long-running top-level DM work showed lifecycle reactions and final replies, but no live draft preview, making work appear stalled.

The merged replacement keeps temporary previews app-authored so Slack can update and delete them, then sends the final text, media, and TTS reply separately with the configured custom username or icon. The replacement superseded the original #107888 implementation while preserving co-author credit.

• Restores live progress visibility for custom-identity Slack agents without requiring configuration changes.
• Preserves Slack edit/delete semantics by separating disposable app-authored previews from customized final delivery.
• Counts as a co-authored merged upstream contribution because the landed commit includes a Co-authored-by trailer and the maintainer linked it back to the original implementation direction.

• Focused Slack streaming and delivery tests covered text, media, and TTS reply paths.
• Live Slack proof confirmed draft creation, draft update, disposable draft deletion, matching custom final identity, and exactly one final post.
• Merged as commit 4233301c after exact-head hosted CI and maintainer review, closing the linked Slack preview issue.