Improve encapsulation of weight_data_type_freq_ (#27292)

### Description
Move weight_data_type_freq_ to private section and add public getter
method GetWeightDataTypeFrequency() to provide controlled read-only
access. Update inference_session.cc to use the new getter instead of
direct member access.

### Motivation and Context
To ensure we follow best practices when accessing class variables.

Author: ashrit-ms

include/onnxruntime/core/graph/graph.h

@@ -779,6 +779,13 @@ class Graph {  // NOLINT(clang-analyzer-optin.performance.Padding): preserve exi
   bool IsSparseInitializer(const std::string& name) const;
 #endif
 
+#if !defined(ORT_MINIMAL_BUILD)
+  /** Gets the frequency count of weight data types in this graph. */
+  gsl::span<const int32_t> GetWeightDataTypeFrequency() const noexcept {
+    return weight_data_type_freq_;
+  }
+#endif
+
   /** Gets an initializer tensor with the provided name.
   @param[out] value Set to the TensorProto* if the initializer is found, or nullptr if not.
   @returns True if found.
@@ -1608,9 +1615,9 @@ class Graph {  // NOLINT(clang-analyzer-optin.performance.Padding): preserve exi
 
   ORT_DISALLOW_COPY_ASSIGNMENT_AND_MOVE(Graph);
 
+ private:
   int32_t weight_data_type_freq_[ONNX_NAMESPACE::TensorProto_DataType_DataType_ARRAYSIZE] = {0};
 
- private:
   void InitializeStateFromModelFileGraphProto();
 
   // Add node with specified <node_proto>.

onnxruntime/core/session/inference_session.cc

@@ -1818,15 +1818,16 @@ static bool ModelHasFP16Inputs(const Graph& graph) {
 #if !defined(ORT_MINIMAL_BUILD)
 [[maybe_unused]] static std::string ModelWeightDataType(const Graph& graph) {
   std::string data_type_list;
+  auto weight_freq = graph.GetWeightDataTypeFrequency();
 
-  for (int i = 0; i < ONNX_NAMESPACE::TensorProto_DataType_DataType_ARRAYSIZE; ++i) {
-    if (graph.weight_data_type_freq_[i] > 0) {
+  for (size_t i = 0; i < weight_freq.size(); ++i) {
+    if (weight_freq[i] > 0) {
       if (!data_type_list.empty()) {
         data_type_list += ", ";
       }
-      data_type_list += TensorProto_DataType_Name(i);
+      data_type_list += TensorProto_DataType_Name(static_cast<int>(i));
       data_type_list += ": ";
-      data_type_list += std::to_string(graph.weight_data_type_freq_[i]);
+      data_type_list += std::to_string(weight_freq[i]);
     }
   }