fix: properly propagate errors in MetricsConfig validation

The previous code had a logic error when checking the result of
FindFieldByName(). It used:
  if (!field.has_value() || !field.value().has_value())

This was incorrect because:
1. If FindFieldByName() returns an error (Result contains an error),
   field.has_value() would be false, but the error was not propagated
2. field.value() returns std::optional<std::reference_wrapper<...>>,
   so the second check was redundant and confusing

Fixed by using ICEBERG_ASSIGN_OR_RAISE to properly propagate any
errors from FindFieldByName(), then checking if the returned optional
contains a value. This matches the pattern used throughout the rest
of the codebase and ensures errors are properly handled.

Author: shangxinli

src/iceberg/metrics_config.cc

@@ -36,8 +36,8 @@ Status MetricsConfig::VerifyReferencedColumns(
     }
     auto field_name =
         std::string_view(key).substr(TableProperties::kMetricModeColumnConfPrefix.size());
-    auto field = schema.FindFieldByName(field_name);
-    if (!field.has_value() || !field.value().has_value()) {
+    ICEBERG_ASSIGN_OR_RAISE(auto field, schema.FindFieldByName(field_name));
+    if (!field.has_value()) {
       return ValidationFailed(
           "Invalid metrics config, could not find column {} from table prop {} in "
           "schema {}",