To support running iPad apps on arm64 Macs, My Mac (Designed for iPad) should show up as a non-ephemeral (ephemeral == device that attach and detach or disappear, like iPhones and simulators) device target.

We originally didn't allow flutter run to avoid confusion with macOS-proper development when that platform was not yet in stable (see comment in #84411).

Now that macOS development has been in stable for awhile, and arm64 Macs are far more prevalent, let's enable this to make it easier for developers to debug their app in whatever way Apple supports.

1. Rename MacOSDesignedForIPadDevice id from 'designed-for-ipad' to 'mac-designed-for-ipad' or similar since visionOS is now another "Designed for iPad" target Support visionOS devices for iOS apps in flutter devices, flutter run, etc. #129638
2. Remove allowDiscovery logic that was disabling discovery for all commands except for attach:
   

   flutter/packages/flutter_tools/lib/src/macos/macos_ipad_device.dart

   Line 126 in eff2e7d

   static bool allowDiscovery = false;

   
   

   flutter/packages/flutter_tools/lib/src/commands/attach.dart

   Lines 215 to 216 in eff2e7d

   	// ARM macOS as an iOS target is hidden, except for attach.
   	MacOSDesignedForIPadDevices.allowDiscovery = true;

3. Until run and drive are implemented (Support flutter run and flutter drive for Mac Designed for iPad apps on arm64 Macs #141328), show an error and toolExit if it's the targeted device for those commands.

Once implemented, validate it appears in the IDE GUI.

Part of #125741