Split out (almost) pure refactoring from pybind/pybind11#5332 (#5334)

PREPARATION for:

PR #5332 — Fix handling of const unique_ptr<T, D> & (do not disown).

Splitting out so that the functional changes under PR #5332 will be more obvious.

The only functional change under this PR is that

```
            assert(custom_deleter_ptr != nullptr);
```

is replaced with:

```
            if (custom_deleter_ptr == nullptr) {
                throw std::runtime_error(
                    std::string("smart_holder::extract_deleter() precondition failure (") + context
                    + ").");
            }
```

include/pybind11/detail/struct_smart_holder.h

@@ -231,6 +231,22 @@ struct smart_holder {
         vptr_del_ptr->armed_flag = armed_flag;
     }
 
+    // Caller is responsible for precondition: ensure_compatible_rtti_uqp_del<T, D>() must succeed.
+    template <typename T, typename D>
+    std::unique_ptr<D> extract_deleter(const char *context) const {
+        auto *gd = std::get_deleter<guarded_delete>(vptr);
+        if (gd && gd->use_del_fun) {
+            const auto &custom_deleter_ptr = gd->del_fun.template target<custom_deleter<T, D>>();
+            if (custom_deleter_ptr == nullptr) {
+                throw std::runtime_error(
+                    std::string("smart_holder::extract_deleter() precondition failure (") + context
+                    + ").");
+            }
+            return std::unique_ptr<D>(new D(std::move(custom_deleter_ptr->deleter)));
+        }
+        return nullptr;
+    }
+
     static smart_holder from_raw_ptr_unowned(void *raw_ptr) {
         smart_holder hld;
         hld.vptr.reset(raw_ptr, [](void *) {});

include/pybind11/detail/type_caster_base.h

@@ -794,22 +794,7 @@ struct load_helper : value_and_holder_helper {
                               "instance cannot safely be transferred to C++.");
         }
 
-        // Temporary variable to store the extracted deleter in.
-        std::unique_ptr<D> extracted_deleter;
-
-        auto *gd = std::get_deleter<pybindit::memory::guarded_delete>(holder().vptr);
-        if (gd && gd->use_del_fun) { // Note the ensure_compatible_rtti_uqp_del<T, D>() call above.
-            // In struct_smart_holder, a custom  deleter is always stored in a guarded delete.
-            // The guarded delete's std::function<void(void*)> actually points at the
-            // custom_deleter type, so we can verify it is of the custom deleter type and
-            // finally extract its deleter.
-            using custom_deleter_D = pybindit::memory::custom_deleter<T, D>;
-            const auto &custom_deleter_ptr = gd->del_fun.template target<custom_deleter_D>();
-            assert(custom_deleter_ptr != nullptr);
-            // Now that we have confirmed the type of the deleter matches the desired return
-            // value we can extract the function.
-            extracted_deleter = std::unique_ptr<D>(new D(std::move(custom_deleter_ptr->deleter)));
-        }
+        std::unique_ptr<D> extracted_deleter = holder().template extract_deleter<T, D>(context);
 
         // Critical transfer-of-ownership section. This must stay together.
         if (self_life_support != nullptr) {