NOT_FOUND for send means: Reported Object was not registered to Server

So we should rather use BAD_REQUEST which means "Undetermined error
occurred"
See : https://www.openmobilealliance.org/release/LightweightM2M/V1_2_1-20221209-A/HTML-Version/OMA-TS-LightweightM2M_Transport-V1_2_1-20221209-A.html#Table-67-4-Response-Codes-Information-Reporting-Interface

leshan-integration-tests/src/test/java/org/eclipse/leshan/integration/tests/send/DynamicIPSendTest.java

@@ -16,7 +16,7 @@
 package org.eclipse.leshan.integration.tests.send;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.eclipse.leshan.core.ResponseCode.NOT_FOUND;
+import static org.eclipse.leshan.core.ResponseCode.BAD_REQUEST;
 import static org.eclipse.leshan.integration.tests.util.Credentials.GOOD_PSK_ID;
 import static org.eclipse.leshan.integration.tests.util.Credentials.GOOD_PSK_KEY;
 import static org.eclipse.leshan.integration.tests.util.Credentials.clientPrivateKey;
@@ -142,7 +142,7 @@ public void can_not_send_if_client_ip_changes(Protocol givenProtocol, String giv
                 Arrays.asList("/3/0/1", "/3/0/2"), 1000);
 
         // it should failed !
-        assertThat(response).hasCode(NOT_FOUND);
+        assertThat(response).hasCode(BAD_REQUEST);
     }
 
     // TODO OSCORE implement a test with OSCORE

leshan-server-cf/src/main/java/org/eclipse/leshan/server/californium/send/SendResource.java

@@ -61,7 +61,7 @@ public void handlePOST(CoapExchange exchange) {
 
         // check we have a registration for this identity
         if (clientProfile == null) {
-            exchange.respond(ResponseCode.NOT_FOUND, "no registration found");
+            exchange.respond(ResponseCode.BAD_REQUEST, "no registration found");
             return;
         }