[coap] use otError in C handlers (#11115)

This commit changes CoAP handlers to use otError in the function or
method definitions. This ensures the definitions are consistent with
the declaration in coap.h.

src/core/thread/dua_manager.cpp

@@ -544,7 +544,7 @@ exit:
 void DuaManager::HandleDuaResponse(void                *aContext,
                                    otMessage           *aMessage,
                                    const otMessageInfo *aMessageInfo,
-                                   Error                aResult)
+                                   otError              aResult)
 {
     static_cast<DuaManager *>(aContext)->HandleDuaResponse(AsCoapMessagePtr(aMessage), AsCoreTypePtr(aMessageInfo),
                                                            aResult);