ssh check mode: rate limit check windows

### What is the issue?

Some tools will open ssh connections regularly (VSCode, for example). When the computer is unattended for extended periods, this can cause a user to come back to hundreds of check mode auth tabs.

This could be rate-limited server side as per @bradfitz

### Steps to reproduce

Set up VSCode ssh to a Tailscale SSH endpoint that has check mode enabled, have a snooze, return to the mess of tabs.

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

_No response_

### OS

_No response_

### OS version

_No response_

### Tailscale version

_No response_

### Bug report

_No response_

Start by reproducing the VSCode SSH check-mode behavior against a Tailscale SSH endpoint and trace the server-side check-mode authentication flow. Done means repeated unattended connections are rate-limited so users do not return to hundreds of authentication tabs, while normal check-mode access still works.

Written by the indexing model from the issue text.