Centralize legacy new document bridge

src/app_core/document_session.h

@@ -112,6 +112,14 @@ struct NewDocumentPlan {
     DocumentFileWriteDecision write_decision = DocumentFileWriteDecision::save_now;
 };
 
+class NewDocumentServices {
+public:
+    virtual ~NewDocumentServices() = default;
+
+    virtual void create_new_document(const NewDocumentPlan& plan) = 0;
+    virtual void prompt_overwrite_new_document(const NewDocumentPlan& plan) = 0;
+};
+
 [[nodiscard]] constexpr ProjectOpenDecision plan_project_open(bool has_unsaved_changes) noexcept
 {
     return has_unsaved_changes
@@ -369,6 +377,22 @@ template <typename ExistsPredicate>
     return pp::foundation::Result<NewDocumentPlan>::success(std::move(plan));
 }
 
+[[nodiscard]] inline pp::foundation::Status execute_new_document_plan(
+    const NewDocumentPlan& plan,
+    NewDocumentServices& services)
+{
+    switch (plan.write_decision) {
+    case DocumentFileWriteDecision::save_now:
+        services.create_new_document(plan);
+        return pp::foundation::Status::success();
+    case DocumentFileWriteDecision::prompt_overwrite:
+        services.prompt_overwrite_new_document(plan);
+        return pp::foundation::Status::success();
+    }
+
+    return pp::foundation::Status::invalid_argument("unknown new document write decision");
+}
+
 [[nodiscard]] inline bool has_legacy_two_character_version_suffix(std::string_view document_name) noexcept
 {
     const auto dot = document_name.rfind('.');