Remove Deprecated Call to FieldDescriptor::is_optional

FieldDescriptor::is_optional is now deprecated and should be replaced with
!is_required() && !is_repeated()
PiperOrigin-RevId: 771098382

fuzztest/internal/domains/protobuf_domain_impl.h

diff --git a/fuzztest/internal/domains/protobuf_domain_impl.h b/fuzztest/internal/domains/protobuf_domain_impl.h
index e65c9b8..a54c6ea 100644
--- a/fuzztest/internal/domains/protobuf_domain_impl.h
+++ b/fuzztest/internal/domains/protobuf_domain_impl.h
@@ -206,7 +206,9 @@
 
 template <typename T>
 Predicate<T> IsOptional() {
-  return [](const T* field) { return field->is_optional(); };
+  return [](const T* field) {
+    return !field->is_required() && !field->is_repeated();
+  };
 }
 
 template <typename T>
@@ -282,7 +284,7 @@
 
   OptionalPolicy GetOptionalPolicy(const FieldDescriptor* field) const {
     FUZZTEST_INTERNAL_CHECK(
-        field->is_optional(),
+        !field->is_required() && !field->is_repeated(),
         "GetOptionalPolicy should apply to optional fields only!");
     std::optional<OptionalPolicy> result =
         GetPolicyValue(optional_policies_, field);
@@ -1803,7 +1805,7 @@
       return true;
     } else if (field->containing_oneof()) {
       return GetOneofFieldPolicy(field) == OptionalPolicy::kWithoutNull;
-    } else if (field->is_optional()) {
+    } else if (!field->is_required() && !field->is_repeated()) {
       return policy_.GetOptionalPolicy(field) == OptionalPolicy::kWithoutNull;
     } else if (field->is_repeated()) {
       return policy_.GetMinRepeatedFieldSize(field).has_value() &&
@@ -1827,7 +1829,7 @@
       return false;
     } else if (field->containing_oneof()) {
       return GetOneofFieldPolicy(field) == OptionalPolicy::kAlwaysNull;
-    } else if (field->is_optional()) {
+    } else if (!field->is_required() && !field->is_repeated()) {
       return policy_.GetOptionalPolicy(field) == OptionalPolicy::kAlwaysNull;
     } else if (field->is_repeated()) {
       return policy_.GetMaxRepeatedFieldSize(field).has_value() &&