fix(opcua): write_data() use 3-arg write_value with data_type_hint + error mapping

Bug fix from review:
- write_data() was calling 2-arg write_value(node_id, value) (bool
  return) instead of 3-arg write_value(node_id, value, data_type_hint)
  (tl::expected return). This meant writes through standard SOVD
  PUT /data/{name} missed the type hint optimization AND collapsed all
  errors to generic 502.
- Now matches handle_plc_operations() and execute_operation() which
  correctly pass entry->data_type and map WriteError codes to
  appropriate HTTP status.

Also: remove redundant `using namespace ros2_medkit_gateway;` inside
the already-open `namespace ros2_medkit_gateway { }` block.

Author: mfaferek93

src/ros2_medkit_plugins/ros2_medkit_opcua/src/opcua_plugin.cpp

@@ -29,8 +29,6 @@
 
 namespace ros2_medkit_gateway {
 
-using namespace ros2_medkit_gateway;
-
 namespace {
 bool is_valid_path_segment(const std::string & s) {
   if (s.empty() || s.size() > 256) {
@@ -747,9 +745,18 @@ tl::expected<nlohmann::json, DataProviderErrorInfo> OpcuaPlugin::write_data(cons
     }
   }
 
-  if (!client_->write_value(entry->node_id, write_val)) {
-    return tl::make_unexpected(DataProviderErrorInfo{DataProviderError::TransportError,
-                                                     "Failed to write value to PLC node: " + entry->node_id_str, 502});
+  auto write_result = client_->write_value(entry->node_id, write_val, entry->data_type);
+  if (!write_result) {
+    auto wcode = write_result.error().code;
+    if (wcode == OpcuaClient::WriteError::TypeMismatch) {
+      return tl::make_unexpected(
+          DataProviderErrorInfo{DataProviderError::InvalidValue, write_result.error().message, 400});
+    }
+    if (wcode == OpcuaClient::WriteError::AccessDenied) {
+      return tl::make_unexpected(DataProviderErrorInfo{DataProviderError::ReadOnly, write_result.error().message, 403});
+    }
+    return tl::make_unexpected(
+        DataProviderErrorInfo{DataProviderError::TransportError, write_result.error().message, 502});
   }
 
   nlohmann::json result;