policy/v2: use bare IPs in autogroup:self DstPorts

Use ip.String() instead of netip.PrefixFrom(ip, ip.BitLen()).String()
when building DstPorts for autogroup:self destinations. This produces
bare IPs like "100.90.199.68" instead of CIDR notation like
"100.90.199.68/32", matching the Tailscale FilterRule wire format.

Updates #2180

hscontrol/policy/v2/filter.go

@@ -3,7 +3,6 @@ package v2
 import (
 	"errors"
 	"fmt"
-	"net/netip"
 	"slices"
 	"strconv"
 	"strings"
@@ -135,6 +134,7 @@ func (pol *Policy) destinationsToNetPortRange(
 				if pref.IsSingleIP() {
 					pr.IP = pref.Addr().String()
 				}
+
 				ret = append(ret, pr)
 			}
 		}
@@ -260,7 +260,7 @@ func (pol *Policy) compileGrantWithAutogroupSelf(
 						for _, port := range ipp.Ports {
 							for _, ip := range n.IPs() {
 								destPorts = append(destPorts, tailcfg.NetPortRange{
-									IP:    netip.PrefixFrom(ip, ip.BitLen()).String(),
+									IP:    ip.String(),
 									Ports: port,
 								})
 							}

hscontrol/policy/v2/filter_test.go

@@ -1846,7 +1846,7 @@ func TestAutogroupSelfWithSpecificUserSource(t *testing.T) {
 		actualDestIPs = append(actualDestIPs, dst.IP)
 	}
 
-	expectedDestIPs := []string{"100.64.0.1/32", "100.64.0.2/32"}
+	expectedDestIPs := []string{"100.64.0.1", "100.64.0.2"}
 	assert.ElementsMatch(t, expectedDestIPs, actualDestIPs)
 
 	node2 := nodes[2].View()