[mle] fix int conversion when setting TLV length (#11727)

This commit resolves a potential unsafe integer conversion warning by
explicitly casting the size of the status sub-TLV to `uint8_t` before
setting the length of the main TLV in
`SendLinkMetricsManagementResponse`.

Additionally, this change enables MLE_LINK_METRICS_SUBJECT_ENABLE`
feature in the `toranj` test configuration, allowing this feature to
be covered under its builds.

src/core/thread/mle.cpp

@@ -2159,7 +2159,7 @@ Error Mle::SendLinkMetricsManagementResponse(const Ip6::Address &aDestination, L
     tlv.SetType(Tlv::kLinkMetricsManagement);
     statusSubTlv.SetType(LinkMetrics::SubTlv::kStatus);
     statusSubTlv.SetLength(sizeof(aStatus));
-    tlv.SetLength(statusSubTlv.GetSize());
+    tlv.SetLength(static_cast<uint8_t>(statusSubTlv.GetSize()));
 
     SuccessOrExit(error = message->Append(tlv));
     SuccessOrExit(error = message->Append(statusSubTlv));

tests/toranj/openthread-core-toranj-config.h

@@ -80,6 +80,8 @@
 
 #define OPENTHREAD_CONFIG_DIAG_ENABLE 1
 
+#define OPENTHREAD_CONFIG_MLE_LINK_METRICS_SUBJECT_ENABLE 1
+
 #define OPENTHREAD_CONFIG_JOINER_ENABLE 1
 
 #define OPENTHREAD_CONFIG_TMF_NETDATA_SERVICE_ENABLE 1