wgengine/magicsock: move UDP relay path discovery to heartbeat() (#16407)

This was previously hooked around direct UDP path discovery /
CallMeMaybe transmission, and related conditions. Now it is subject to
relay-specific considerations.

Updates tailscale/corp#27502

Signed-off-by: Jordan Whited <jordan@tailscale.com>

wgengine/magicsock/endpoint_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 	"time"
 
+	"tailscale.com/tailcfg"
 	"tailscale.com/types/key"
 )
 
@@ -323,3 +324,44 @@ func Test_endpoint_maybeProbeUDPLifetimeLocked(t *testing.T) {
 		})
 	}
 }
+
+func Test_epAddr_isDirectUDP(t *testing.T) {
+	vni := virtualNetworkID{}
+	vni.set(7)
+	tests := []struct {
+		name string
+		ap   netip.AddrPort
+		vni  virtualNetworkID
+		want bool
+	}{
+		{
+			name: "true",
+			ap:   netip.MustParseAddrPort("192.0.2.1:7"),
+			vni:  virtualNetworkID{},
+			want: true,
+		},
+		{
+			name: "false derp magic addr",
+			ap:   netip.AddrPortFrom(tailcfg.DerpMagicIPAddr, 0),
+			vni:  virtualNetworkID{},
+			want: false,
+		},
+		{
+			name: "false vni set",
+			ap:   netip.MustParseAddrPort("192.0.2.1:7"),
+			vni:  vni,
+			want: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			e := epAddr{
+				ap:  tt.ap,
+				vni: tt.vni,
+			}
+			if got := e.isDirect(); got != tt.want {
+				t.Errorf("isDirect() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}