[mac-frame] set mCslPresent on TxFrame when CSL IE is added (open…

…thread#10939)

This commit ensures that the `mCslPresent` flag is set on `TxFrame`
when the CSL IE header is appended to the frame. This addresses an
issue introduced in openthread#10692 where this flag can remain unset.

src/core/mac/mac_frame.cpp

@@ -232,6 +232,7 @@ void TxFrame::Info::PrepareHeadersIn(TxFrame &aTxFrame) const
     {
         builder.Append<HeaderIe>()->Init(CslIe::kHeaderIeId, sizeof(CslIe));
         builder.Append<CslIe>();
+        aTxFrame.SetCslIePresent(true);
     }
 #endif