macos: restarting GUI app while tailscale is disconnected leaks a tun interface

### What is the issue?

Restarting the Tailscale GUI/menubar app on macos (network extension variant) while Tailscale is _disconnected_ leads to the creation of a new tun interface each time the GUI is started. This does not reproduce if Tailscale is connected when the GUI is restarted.

I repro'd this on two machines, one on stable (1.102.2) and other unstable (1.103.72)

```
$ ifconfig | grep utun | grep -c mtu
9
# restart gui
$ ifconfig | grep utun | grep -c mtu
10
# restart gui
$ ifconfig | grep utun | grep -c mtu
11
```

### Steps to reproduce

1. Disable Tailscale toggle in macos menubar
2. `ifconfig | grep utun | grep -c mtu`
3. Quit Tailscale from macos menubar
4. Open Tailscale again
5. `ifconfig | grep utun | grep -c mtu`

### Are there any recent changes that introduced the issue?

_No response_

### OS

macOS

### OS version

26.5.2, 26.5.1

### Tailscale version

1.102.2, 1.103.72

### Other software

_No response_

### Bug report

stable: `BUG-0b1ae8a6012765ea2073397b11afd60d1f7c32becc57ce0be59e4bcd08d57a0c-20260810044620Z-f1030233d00f0a2a`

unstable: `BUG-0ed9a5dd8b7e4b934aebf17862dc43e54f374d6698cbab940d4bce2e37243ca8-20260810175027Z-9eee9818d3d912ad`

Reproduce the leak with the macOS menubar GUI, the disconnected Tailscale state, and the two ifconfig commands described in the issue. Trace the macOS network-extension lifecycle across quitting and reopening the GUI; done means repeated restarts while disconnected no longer increase the utun count, while the connected case remains unaffected.

Written by the indexing model from the issue text.