[cli] Update command "borderagent discovery" to allow network interfa…

…ce selection for mDNS binding.

Within the OTBR practice application, the BR host will have several
interfaces. When users want to discover the border agent using mDNS,
they'll need to choose the specific interface the border agent is bound
to.

This change introduces a new option for the CLI command `borderagent
dicover`. Users can now specify a network interface using the syntax
`borderagent discovery <timeout> <network interface>`. The chosen
interface will be used for mDNS binding through socket options.

src/app/br_discover.cpp

@@ -53,8 +53,9 @@ Error DiscoverBorderAgent(BorderAgentHandler aBorderAgentHandler, size_t aTimeou
     int socket = mdns_socket_open_ipv4();
     VerifyOrExit(socket >= 0, error = ERROR_IO_ERROR("failed to open mDNS IPv4 socket"));
 
-    if (aNetIf != "" && setsockopt(socket, SOL_SOCKET, SO_BINDTODEVICE, &aNetIf[0], sizeof(aNetIf)) < 0) {
-         ExitNow(error = ERROR_IO_ERROR("failed to bind network interface: {}", aNetIf));
+    if (aNetIf != "" && setsockopt(socket, SOL_SOCKET, SO_BINDTODEVICE, &aNetIf[0], sizeof(aNetIf)) < 0)
+    {
+         ExitNow(error = ERROR_SOCKET_BIND_ERROR("failed to bind network interface: {}", aNetIf));
     }
 
     if (mdns_query_send(socket, kMdnsQueryType, kServiceName, strlen(kServiceName), buf, sizeof(buf)) != 0)

src/app/cli/interpreter.cpp

@@ -1788,6 +1788,7 @@ Interpreter::Value Interpreter::ProcessBorderAgent(const Expression &aExpr)
     if (CaseInsensitiveEqual(aExpr[1], "discover"))
     {
         uint64_t timeout = 4000;
+        std::string netIf = "";
 
         if (aExpr.size() >= 3)
         {