fix: truncate long text in Slack approval card updates to avoid invalid_blocks

[Yuki-14544869]

Problem

Slack's chat.update API rejects section blocks with text longer than 3000 characters (invalid_blocks / must be less than 3001 characters).

When code execution approval cards contain long code previews, the _handle_approval_action and _handle_slash_confirm_action handlers preserve the original text in updated_blocks. This can exceed the 3000-char limit and cause the chat_update to fail silently — leaving the approval buttons visible even after the user has successfully clicked them.

Reproduction

1. Send a code execution approval request with a code preview >3000 chars
2. Click any approval button (approve/deny)
3. The approval is resolved (agent unblocks) but the buttons remain visible
4. Gateway logs show: Failed to update approval message: invalid_blocks

Fix

Truncate original_text to 2900 characters + ellipsis when rebuilding the updated blocks in both handlers.

Changed files: gateway/platforms/slack.py (2 lines)

Logs

WARNING gateway.platforms.slack: [Slack] Failed to update approval message: The request to the Slack API failed. (url: https://slack.com/api/chat.update, status: 200)
The server responded with: {'ok': False, 'error': 'invalid_blocks', '1': 'must be less than 3001 characters [json-pointer:/blocks/0/text/text]'}

[alt-glitch]

Related to #35389 (competing fix, same approach) and #27441 (overloaded PR with same fix buried in 36 files). All three address the same Slack Block Kit 3000-char section text limit on approval card updates.