Two fixes for Discord exec approval:
1. Register /approve and /deny as native Discord slash commands so they
appear in Discord's command picker (autocomplete). Previously they
were only handled as text commands, so users saw 'no commands found'
when typing /approve.
2. Wire up the existing ExecApprovalView button UI (was dead code):
- ExecApprovalView now calls resolve_gateway_approval() to actually
unblock the waiting agent thread when a button is clicked
- Gateway's _approval_notify_sync() detects adapters with
send_exec_approval() and routes through the button UI
- Added 'Allow Session' button for parity with /approve session
- send_exec_approval() now accepts session_key and metadata for
thread support
- Graceful fallback to text-based /approve prompt if button send fails
Also updates test mocks to include grey/secondary ButtonStyle and
purple Color (used by new button styles).
- Download and cache .pdf, .docx, .xlsx, .pptx attachments locally
instead of passing expiring CDN URLs to the agent
- Inject .txt and .md content (≤100 KB) into event.text so the agent
sees file content without needing to fetch the URL
- Add 20 MB size guard and SUPPORTED_DOCUMENT_TYPES allowlist
- Fix: unsupported types (.zip etc.) no longer get MessageType.DOCUMENT
- Add 9 unit tests in test_discord_document_handling.py
Mirrors the Slack implementation from PR #784. Discord CDN URLs are
publicly accessible so no auth header is needed (unlike Slack).
Co-authored-by: Dilee <uzmpsk.dilekakbas@gmail.com>