Fix type annotation in upload sync path prefix calculation

Replace incorrect use of reduce() with direct os.path.commonprefix() call.
The original code used reduce() incorrectly - os.path.commonprefix() already
accepts a list of paths and returns the longest common prefix.

This removes one type: ignore comment and improves code clarity.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: yarikoptic

dandi/upload.py

@@ -4,7 +4,6 @@
 from collections.abc import Iterator, Sequence
 from contextlib import ExitStack
 from enum import Enum
-from functools import reduce
 import io
 import os.path
 from pathlib import Path
@@ -493,7 +492,7 @@ def upload_agg(*ignored: Any) -> str:
             for p in paths:
                 rp = os.path.relpath(p, dandiset.path)
                 relpaths.append("" if rp == "." else rp)
-            path_prefix = reduce(os.path.commonprefix, relpaths)  # type: ignore[arg-type]
+            path_prefix = os.path.commonprefix(relpaths)
             to_delete = []
             for asset in remote_dandiset.get_assets_with_path_prefix(path_prefix):
                 if any(