feature/conn25: extend assignment expiry on use

When we use assigned addresses in response to a DNS request, extend the
expiry on the assignment.

Updates tailscale/corp#39975

Signed-off-by: Fran Bull <fran@tailscale.com>

feature/conn25/conn25.go

@@ -755,6 +755,7 @@ func (c *client) reserveAddresses(appName string, domain dnsname.FQDN, dst netip
 	c.mu.Lock()
 	defer c.mu.Unlock()
 	if existing, ok := c.assignments.lookupByDomainDst(domain, dst); ok {
+		c.assignments.updateFromTTL(existing, ttl)
 		return existing, nil
 	}
 
@@ -766,7 +767,7 @@ func (c *client) reserveAddresses(appName string, domain dnsname.FQDN, dst netip
 	now := c.assignments.clock.Now()
 	for range 10 {
 		a := c.assignments.popExpired(now)
-		if !a.isValid() {
+		if a == nil {
 			break
 		}
 		if a.is4() {