Observatory: Clear removed outbounds (#5876)

* fix: prune stale observatory status

* More readable

Refactor observer to clear removed outbounds instead of updating status. Introduced slices package for improved outbound checking.

---------

Co-authored-by: 风扇滑翔翼 <Fangliding.fshxy@outlook.com>

app/observatory/observer_test.go

@@ -0,0 +1,64 @@
+package observatory
+
+import "testing"
+
+func TestObserverUpdateStatusPrunesStaleOutbounds(t *testing.T) {
+	observer := &Observer{
+		status: []*OutboundStatus{
+			{
+				OutboundTag:     "keep",
+				Alive:           true,
+				Delay:           42,
+				LastErrorReason: "",
+				LastSeenTime:    111,
+				LastTryTime:     222,
+			},
+			{
+				OutboundTag:     "drop",
+				Alive:           false,
+				Delay:           99999999,
+				LastErrorReason: "probe failed",
+				LastSeenTime:    333,
+				LastTryTime:     444,
+			},
+		},
+	}
+
+	observer.clearRemovedOutbounds([]string{"keep"})
+
+	if len(observer.status) != 1 {
+		t.Fatalf("expected 1 status after pruning, got %d", len(observer.status))
+	}
+
+	got := observer.status[0]
+	if got.OutboundTag != "keep" {
+		t.Fatalf("expected remaining status for keep, got %q", got.OutboundTag)
+	}
+	if !got.Alive {
+		t.Fatal("expected remaining status to preserve Alive field")
+	}
+	if got.Delay != 42 {
+		t.Fatalf("expected remaining status to preserve Delay, got %d", got.Delay)
+	}
+	if got.LastSeenTime != 111 {
+		t.Fatalf("expected remaining status to preserve LastSeenTime, got %d", got.LastSeenTime)
+	}
+	if got.LastTryTime != 222 {
+		t.Fatalf("expected remaining status to preserve LastTryTime, got %d", got.LastTryTime)
+	}
+}
+
+func TestObserverUpdateStatusClearsWhenNoOutboundsRemain(t *testing.T) {
+	observer := &Observer{
+		status: []*OutboundStatus{
+			{OutboundTag: "drop-1"},
+			{OutboundTag: "drop-2"},
+		},
+	}
+
+	observer.clearRemovedOutbounds(nil)
+
+	if len(observer.status) != 0 {
+		t.Fatalf("expected all statuses to be removed, got %d", len(observer.status))
+	}
+}