AdminModule: don't return the device private key to remote config reads (#11030)

A SECURITY_CONFIG get_config response copied config.security verbatim, so a remote
request was answered with the device identity private_key and sent over the air.
Only the local owner needs it, for backup.

Zero private_key in the SECURITY_CONFIG response when the request is remote
(from != 0); the local BLE/USB/TCP path (from == 0) still receives it. public_key
and admin_key are public and stay as they were.

test/support/AdminModuleTestShim.h

@@ -8,11 +8,15 @@ class AdminModuleTestShim : public AdminModule
 {
   public:
     using AdminModule::checkPassKey; // session-key gate seam (see test_admin_session_repro)
+    using AdminModule::handleGetConfig;
     using AdminModule::handleReceivedProtobuf;
     using AdminModule::handleSetConfig;
     using AdminModule::handleSetModuleConfig;
     using AdminModule::setPassKey;
 
+    // Peek at the reply a handler queued, before drainReply() releases it.
+    meshtastic_MeshPacket *reply() { return myReply; }
+
     // With an "open edit transaction" saveChanges() is a pure no-op: no reloadConfig/saveToDisk/reboot.
     void deferSaves() { hasOpenEditTransaction = true; }