ipn/{localapi, ipnlocal}: forget the prior exit node when localAPI is used to zero the ExitNodeID (#11681)

Updates tailscale/corp#18724

When localAPI clients directly set ExitNodeID to "", the expected behaviour is that the prior exit node also gets zero'd - effectively setting the UI state back to 'no exit node was ever selected'

The IntenalExitNodePrior has been changed to be a non-opaque type, as it is read by the UI to render the users last selected exit node, and must be concrete. Future-us can either break this, or deprecate it and replace it with something more interesting.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

ipn/ipn_clone.go

@@ -42,7 +42,7 @@ var _PrefsCloneNeedsRegeneration = Prefs(struct {
 	AllowSingleHosts       bool
 	ExitNodeID             tailcfg.StableNodeID
 	ExitNodeIP             netip.Addr
-	InternalExitNodePrior  string
+	InternalExitNodePrior  tailcfg.StableNodeID
 	ExitNodeAllowLANAccess bool
 	CorpDNS                bool
 	RunSSH                 bool