fix: recurse through pasted block data to replace static paths (#36723)

Fixes an error that is triggered when trying to copy/pasting Drag-and-drop block into Courses.

When a block is pasted into a Course or Library, we perform a search/replace on block's data to replace any old static URLs with their new path. Ordinary blocks like HTML and Video have a simple string of data where static URLs may live, but DnDv2 blocks have a dict of data which can contain other dicts and lists of strings that need to be modified. Other XBlocks may have similarly complex structures, and so this fix will resolve them as well.

This fixes the issue by recursing into the data structure to locate all the strings where replacements may need to be made.

This fix helps Course Authors use content staging in their Courses.

Author: pomegranited

cms/djangoapps/contentstore/helpers.py

@@ -315,7 +315,8 @@ def _insert_static_files_into_downstream_xblock(
     if hasattr(downstream_xblock, "data") and substitutions:
         data_with_substitutions = downstream_xblock.data
         for old_static_ref, new_static_ref in substitutions.items():
-            data_with_substitutions = data_with_substitutions.replace(
+            data_with_substitutions = _replace_strings(
+                data_with_substitutions,
                 old_static_ref,
                 new_static_ref,
             )
@@ -325,6 +326,26 @@ def _insert_static_files_into_downstream_xblock(
     return notices
 
 
+def _replace_strings(obj: dict | list | str, old_str: str, new_str: str):
+    """
+    Replacing any instances of the given `old_str` string with `new_str` in any strings found in the the given object.
+
+    Returns the updated object.
+    """
+    if isinstance(obj, dict):
+        for key, value in obj.items():
+            obj[key] = _replace_strings(value, old_str, new_str)
+
+    elif isinstance(obj, list):
+        for index, item in enumerate(obj):
+            obj[index] = _replace_strings(item, old_str, new_str)
+
+    elif isinstance(obj, str):
+        return obj.replace(old_str, new_str)
+
+    return obj
+
+
 def import_staged_content_from_user_clipboard(parent_key: UsageKey, request) -> tuple[XBlock | None, StaticFileNotices]:
     """
     Import a block (along with its children and any required static assets) from