Please address the comments from this code review:
## Overall Comments
- Instead of ordering pages by `path` to enforce parent-first order, use the model’s MPTT attributes (e.g. `tree_id` and `lft`) for deterministic tree traversal.
- Extract the PageUrl copy logic into a shared helper or into the Page model’s copy method to avoid duplication and improve maintainability.
- Iterate over all related PageUrl instances (`page.urls.all()`) so that any URL aliases are also copied, not just the primary URL.

## Individual Comments

### Comment 1
<location> `cms/management/commands/subcommands/copy.py:98` </location>
<code_context>
+                        page.update_languages(page.get_languages() + [to_lang])
+
+                    # copy PageUrls - inspired from pagemodels.Page.copy() - possibly refactorable
+                    page_url = page.urls.get(language=from_lang)
+                    parent_page = page.parent
+
+                    new_url = model_to_dict(page_url)
</code_context>

<issue_to_address>
No error handling for missing PageUrl in source language.

Accessing page.urls.get(language=from_lang) without checking for existence can cause exceptions. Please add error handling to manage missing PageUrl cases.
</issue_to_address>

### Comment 2
<location> `cms/management/commands/subcommands/copy.py:114` </location>
<code_context>
+                        path = page_url.slug
+
+                    new_url["slug"] = get_available_slug(site, path, to_lang)
+                    new_url["path"] = '%s/%s' % (base, new_url["slug"]) if base else new_url["slug"]
+                    PageUrl.objects.with_user(user).create(**new_url)
+
</code_context>

<issue_to_address>
Path construction may not handle leading/trailing slashes or empty base correctly.

Normalize base and slug to prevent double slashes or malformed paths when concatenating.

Suggested implementation:

```python
                        base = parent_page.get_path(to_lang)
                        # Normalize base and slug to avoid double slashes
                        base_norm = base.strip('/') if base else ''
                        slug_norm = page_url.slug.strip('/')
                        path = '%s/%s' % (base_norm, slug_norm) if base_norm else slug_norm
                    else:
                        base_norm = ''
                        slug_norm = page_url.slug.strip('/')
                        path = slug_norm

                    new_url["slug"] = get_available_slug(site, path, to_lang)

```

```python
                    new_url["slug"] = get_available_slug(site, path, to_lang)
                    # Normalize base and slug for the final path as well
                    final_base = base.strip('/') if base else ''
                    final_slug = new_url["slug"].strip('/')
                    new_url["path"] = '%s/%s' % (final_base, final_slug) if final_base else final_slug
                    PageUrl.objects.with_user(user).create(**new_url)

```
</issue_to_address>