Use safe_numel() in wasm bindings

The wasm bindings' assert_valid_numel() helper previously computed numel without overflow detection, letting a malicious tensor shape wrap to a small value that bypassed the data-size check. Replace with safe_numel() (returns Result<ssize_t>) and propagate the error via THROW_IF_ERROR as a JavaScript exception, matching the rest of the wasm error plumbing.

Authored with Claude.

Differential Revision: [D102082911](https://our.internmc.facebook.com/intern/diff/D102082911/)

[ghstack-poisoned]

Author: Github Executorch

extension/wasm/wasm_bindings.cpp

@@ -84,20 +84,23 @@ inline void js_array_push(val_array<T>& array, const T& value) {
   _(float, Float)                           \
   _(int64_t, Long)
 
-inline ssize_t compute_expected_numel(
+inline ::executorch::runtime::Result<ssize_t> compute_expected_numel(
     const std::vector<torch::executor::Tensor::SizesType>& sizes) {
-  return executorch::aten::compute_numel(sizes.data(), sizes.size());
+  return executorch::aten::safe_numel(sizes.data(), sizes.size());
 }
 
 template <typename T>
 inline void assert_valid_numel(
     const std::vector<T>& data,
     const std::vector<torch::executor::Tensor::SizesType>& sizes) {
   auto computed_numel = compute_expected_numel(sizes);
+  THROW_IF_ERROR(
+      computed_numel.error(),
+      "Invalid tensor sizes: numel computation failed");
   THROW_IF_FALSE(
-      data.size() >= computed_numel,
+      data.size() >= static_cast<size_t>(computed_numel.get()),
       "Required %ld elements, given %ld",
-      computed_numel,
+      computed_numel.get(),
       data.size());
 }