[dataset] returns OT_ERROR_REJECTED when MGMT_SET is rejected by lead…

…er (openthread#9582)

The current code won't return a failure error code when a MGMT_SET
request is rejected by the leader, so the client doesn't know whether
the operation succeed or not.

This commit fixes this issue by converting the REJECTED state to the
OT_ERROR_REJECTED error code which is propagated back via the
otDatasetMgmtSetCallback callback.

src/core/meshcop/dataset_manager.cpp

@@ -312,15 +312,18 @@ void DatasetManager::HandleMgmtSetResponse(Coap::Message *aMessage, const Ip6::M
     OT_UNUSED_VARIABLE(aMessageInfo);
 
     Error   error;
-    uint8_t state;
+    uint8_t state = StateTlv::kPending;
 
     SuccessOrExit(error = aError);
     VerifyOrExit(Tlv::Find<StateTlv>(*aMessage, state) == kErrorNone && state != StateTlv::kPending,
                  error = kErrorParse);
+    if (state == StateTlv::kReject)
+    {
+        error = kErrorRejected;
+    }
 
 exit:
-    LogInfo("MGMT_SET finished: %s",
-            error == kErrorNone ? StateTlv::StateToString(static_cast<StateTlv::State>(state)) : ErrorToString(error));
+    LogInfo("MGMT_SET finished: %s", error == kErrorNone ? "Accepted" : ErrorToString(error));
 
     mMgmtPending = false;