[indirect-sender] reset message timestamp on child mode change to non…

…-sleepy (openthread#10979)

This commit resets (to now) the timestamp of queued messages for a
previously sleepy child when the child mode changes to non-sleepy.
This ensures that delay-aware queue management is correctly applied
to these messages and avoids them being dropped immediately.

src/core/thread/indirect_sender.cpp

@@ -205,6 +205,7 @@ void IndirectSender::HandleChildModeChange(Child &aChild, Mle::DeviceMode aOldMo
             {
                 message.GetIndirectTxChildMask().Remove(childIndex);
                 message.SetDirectTransmission();
+                message.SetTimestampToNow();
             }
         }