fix: parts.metadata use md_nesting as key (#1395)

Author: ffffffff0

aperag/docparser/chunking.py

@@ -110,7 +110,7 @@ def _to_groups(self, parts: list[Part]) -> list[Group]:
             if not part.content:
                 continue
 
-            nesting = part.metadata.get("nesting", 0)
+            nesting = part.metadata.get("md_nesting", 0)
             title_level = 0
             title = ""
             if hasattr(part, "level"):  # TitlePart