fix(slack): truncate section text when updating approval messages

[b32n]

Fixes #62054

What

Slack chat.update fails with invalid_blocks when updating approval or slash-confirm messages after a button click — the section block text exceeds Slack's 3000-character limit.

Why

send_exec_approval and send_slash_confirm budget content on send, but _handle_approval_action and _handle_slash_confirm_action copy original_text verbatim when rebuilding blocks after a button click.

Fix

Add _fit_slack_section_text() to clamp section mrkdwn to 3000 chars before chat.update.

How to test

pytest tests/gateway/test_slack_approval_buttons.py::TestSlackApprovalAction -v

Manual repro (production symptom):

1. Trigger a command approval with a large execute_code script.
2. Click "Allow Once".
3. Message should update to show the decision; buttons removed.

Previously: invalid_blocks / must be less than 3001 characters [json-pointer:/blocks/0/text/text] — approval still resolved but UI did not update.

Platforms tested

• macOS (pytest)

[alt-glitch]

This was generated by AI during triage.

Duplicate of #53701 (earliest-open canonical) — same code-site and mechanism: clamp the echoed original_text to Slack's 3000-char section limit before chat.update in _handle_approval_action / _handle_slash_confirm_action. #61697 is already marked a duplicate of #53701 via this same mechanism. Both are the post-decision counterpart to the merged send-time fix #43444 (which did not cover the chat.update-rebuild path); related to the issue #53693 / #62054.

[b32n]

Closing this PR as it's a duplicate.