fix(discord): register /approve and /deny slash commands, wire up button-based approval UI (#4800)

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).

gateway/run.py

@@ -5851,10 +5851,39 @@ class GatewayRunner:
             from tools.approval import register_gateway_notify, unregister_gateway_notify
 
             def _approval_notify_sync(approval_data: dict) -> None:
-                """Send the approval request to the user from the agent thread."""
+                """Send the approval request to the user from the agent thread.
+
+                If the adapter supports interactive button-based approvals
+                (e.g. Discord's ``send_exec_approval``), use that for a richer
+                UX.  Otherwise fall back to a plain text message with
+                ``/approve`` instructions.
+                """
                 cmd = approval_data.get("command", "")
-                cmd_preview = cmd[:200] + "..." if len(cmd) > 200 else cmd
                 desc = approval_data.get("description", "dangerous command")
+
+                # Prefer button-based approval when the adapter supports it.
+                # Check the *class* for the method, not the instance — avoids
+                # false positives from MagicMock auto-attribute creation in tests.
+                if getattr(type(_status_adapter), "send_exec_approval", None) is not None:
+                    try:
+                        asyncio.run_coroutine_threadsafe(
+                            _status_adapter.send_exec_approval(
+                                chat_id=_status_chat_id,
+                                command=cmd,
+                                session_key=_approval_session_key,
+                                description=desc,
+                                metadata=_status_thread_metadata,
+                            ),
+                            _loop_for_step,
+                        ).result(timeout=15)
+                        return
+                    except Exception as _e:
+                        logger.warning(
+                            "Button-based approval failed, falling back to text: %s", _e
+                        )
+
+                # Fallback: plain text approval prompt
+                cmd_preview = cmd[:200] + "..." if len(cmd) > 200 else cmd
                 msg = (
                     f"⚠️ **Dangerous command requires approval:**\n"
                     f"```\n{cmd_preview}\n```\n"