fix: Escape CDATA special char on xblock serialize (#33239)

This fixes a bug in xblock serialization when trying to copy a
unit/component that contains an HTML xblock with the characters "]]>"
which is a special character in CDATA.

openedx/core/lib/xblock_serializer/block_serializer.py

@@ -112,7 +112,10 @@ class XBlockSerializer:
             olx_node.attrib["editor"] = block.editor
         if block.use_latex_compiler:
             olx_node.attrib["use_latex_compiler"] = "true"
-        olx_node.text = etree.CDATA("\n" + block.data + "\n")
+
+        # Escape any CDATA special chars
+        escaped_block_data = block.data.replace("]]>", "]]>")
+        olx_node.text = etree.CDATA("\n" + escaped_block_data + "\n")
         return olx_node