fix: sync MTProto startup and egress fixes

internal/postresponse/callbacks_test.go

@@ -0,0 +1,28 @@
+package postresponse
+
+import (
+	"context"
+	"sync/atomic"
+	"testing"
+)
+
+func TestTakeTransfersCallbacksAndRunsOnce(t *testing.T) {
+	ctx := WithCallbacks(context.Background())
+	var calls atomic.Int32
+	if !Register(ctx, func() { calls.Add(1) }) || !Register(ctx, func() { calls.Add(1) }) {
+		t.Fatal("register callbacks")
+	}
+	run := Take(ctx)
+	if run == nil {
+		t.Fatal("Take returned no callback")
+	}
+	Run(ctx)
+	if got := calls.Load(); got != 0 {
+		t.Fatalf("callbacks remained attached after Take: %d", got)
+	}
+	run()
+	run()
+	if got := calls.Load(); got != 2 {
+		t.Fatalf("transferred callbacks ran %d times, want 2 total", got)
+	}
+}