Fix merge_index dropping nested directories from shard paths

[discobot]

Description of changes:

merge_index drops intermediate directories from shard paths in the merged index.json when
partitions are nested more than one level under the root. Root cause: _merge_index_from_list
(streaming/base/util.py) rebuilds each merged basename as
os.path.join(os.path.basename(p.parent), basename) — exactly one directory level of the
partition index's path — so anything between the merge root and the partition is silently
dropped.

This PR resolves each partition's directory relative to the merge root instead, via a new
_get_partition_dirname helper that compares the partition index url against out (local part
against local root, remote against remote, same bucket only) and takes os.path.relpath on the
url paths. Partitions that don't reside under out (arbitrary index_file_urls) fall back to
the previous single-level behavior, so existing one-level layouts are unaffected.

Testing:

• New test_merge_index_from_root_local_nested writes 4 MDS partitions at
  out/group{1,2}/subdir{1,2}, merges, and asserts the merged basenames keep the full relative
  paths and that a StreamingDataset on the merged root iterates all samples. It fails on main
  ("expected 4 shard files but got 2") and passes with this change.
• New test_get_partition_dirname covers local, remote, tuple, root-level, cross-bucket, and
  outside-root fallback cases.
• All pre-existing tests/test_util.py tests (incl. the spark-based
  test_merge_index_from_list_local / test_merge_index_from_root_local matrices) and
  tests/base/converters/test_dataframe_to_mds.py pass locally.

Issue #, if available:

• Fixes merge_index drops full directory path in merged index.json, causing broken shard references and 404s #959

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