Return the secret sentinel for remote admin config gets (#11093)

writeSecret is a setter, so calling it on the NETWORK_CONFIG get path was a no-op: the buffer
already holds the stored psk, never the sentinel. MQTT_CONFIG returned the broker password
verbatim.

Both get paths now return secretReserved when req.from != 0, and the matching set paths call
writeSecret so a read-modify-write round trip keeps the stored value.

test/support/AdminModuleTestShim.h

@@ -9,6 +9,7 @@ class AdminModuleTestShim : public AdminModule
   public:
     using AdminModule::checkPassKey; // session-key gate seam (see test_admin_session_repro)
     using AdminModule::handleGetConfig;
+    using AdminModule::handleGetModuleConfig;
     using AdminModule::handleReceivedProtobuf;
     using AdminModule::handleSetConfig;
     using AdminModule::handleSetModuleConfig;