control,health,ipn: move IP forwarding check to health tracker (#19007)

Currently IP forwarding health check is done on sending MapRequests.

Move ip forwarding to the health service to gain the benefits
of the health tracker and perodic monitoring out of band from
the MapRequest path. ipnlocal now provides a closure to
the health service to provide the check if forwarding is broken.

Removed `skipIPForwardingCheck` from controlclient/direct.go,
it wasn't being used as the comments describe it, that check
has moved to ipnlocal for the closure to the health tracker.

Updates #18976

Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>

health/health_test.go

@@ -999,3 +999,86 @@ func TestCurrentStateETagWarnable(t *testing.T) {
 		}
 	})
 }
+
+func TestIPForwardingState(t *testing.T) {
+	tests := []struct {
+		name          string
+		checkFunc     func() bool // nil means no check function
+		wantUnhealthy bool
+	}{
+		{
+			name:          "broken",
+			checkFunc:     func() bool { return true },
+			wantUnhealthy: true,
+		},
+		{
+			name:          "healthy",
+			checkFunc:     func() bool { return false },
+			wantUnhealthy: false,
+		},
+		{
+			name:          "no_check_function",
+			checkFunc:     nil,
+			wantUnhealthy: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			bus := eventbus.New()
+			tr := NewTracker(bus)
+			defer bus.Close()
+
+			tr.SetIPNState("Running", true)
+			tr.SetIPForwardingCheck(tt.checkFunc)
+
+			tr.mu.Lock()
+			tr.updateBuiltinWarnablesLocked()
+			tr.mu.Unlock()
+
+			got := tr.IsUnhealthy(ipForwardingWarnable)
+			if got != tt.wantUnhealthy {
+				t.Errorf("IsUnhealthy(ipForwardingWarnable) = %v, want %v", got, tt.wantUnhealthy)
+			}
+		})
+	}
+
+	// Test state transitions
+	t.Run("transitions", func(t *testing.T) {
+		bus := eventbus.New()
+		tr := NewTracker(bus)
+		defer bus.Close()
+
+		tr.SetIPNState("Running", true)
+
+		// Start broken
+		tr.SetIPForwardingCheck(func() bool { return true })
+		tr.mu.Lock()
+		tr.updateBuiltinWarnablesLocked()
+		tr.mu.Unlock()
+
+		if !tr.IsUnhealthy(ipForwardingWarnable) {
+			t.Fatal("expected IP forwarding to be unhealthy initially")
+		}
+
+		// Transition to healthy
+		tr.SetIPForwardingCheck(func() bool { return false })
+		tr.mu.Lock()
+		tr.updateBuiltinWarnablesLocked()
+		tr.mu.Unlock()
+
+		if tr.IsUnhealthy(ipForwardingWarnable) {
+			t.Fatal("expected IP forwarding to be healthy after transition")
+		}
+
+		// Transition to nil (should stay healthy)
+		tr.SetIPForwardingCheck(nil)
+		tr.mu.Lock()
+		tr.updateBuiltinWarnablesLocked()
+		tr.mu.Unlock()
+
+		if tr.IsUnhealthy(ipForwardingWarnable) {
+			t.Fatal("expected IP forwarding to be healthy after clearing check")
+		}
+	})
+}