Summary

Adds three primitives to LanceFileSession (lance.file), each a binding-exposure of an existing object_store capability (Rust → PyO3 → Python). They let callers route listing, deletion, and ranged reads through the dataset's object_store session instead of PyArrow's AzureFileSystem.

APIs added

1. list_with_delimiter(path=None) -> ListResult

Non-recursive, path-segment-delimited list of a single directory level. Unlike the existing recursive list(), it returns only the immediate children of path: child "directories" as common_prefixes and direct child files as objects (a ListResult dataclass), all session-relative.

result = session.list_with_delimiter("subdir")
result.common_prefixes  # immediate child "directories"
result.objects          # immediate child files

• lance-io: add ObjectStore::list_with_delimiter(prefix) -> Result<ListResult>.

2. delete_file(path) -> None

Delete an object by session-relative path. Deleting a missing path is a no-op (idempotent) rather than an error, so it is safe on best-effort cleanup paths.

session.delete_file("subdir/checkpoint.lance")  # removes it
session.delete_file("already_gone.lance")        # no-op, no error

3. read_range(path, offset, length) -> bytes

Single ranged read of a byte slice from a file. Reading a missing object raises OSError, consistent with download_file.

data = session.read_range("data/file-0.lance", offset=1024, length=256)  # -> bytes

Why this matters

These remove the remaining PyArrow AzureFileSystem / filesystem_from_uri dependencies from geneva's checkpoint and range-blob paths:

• The recursive-only list() forced the flat-namespace bf-dir enumeration fallback to scan the whole checkpoint subtree; list_with_delimiter makes that fallback bounded again.
• LanceFileSession had no delete, so checkpoint purge fell back to PyArrow and silently became a no-op on flat / unreachable-DFS accounts; delete_file makes purge functional and blob-only.
• The range-blob reader's last PyArrow handle is replaced by read_range.

Testing

• test_session_list_with_delimiter — non-recursive semantics (base level returns immediate files + child dirs but not their contents; subdir descends one level; trailing-slash equivalence; empty for a missing prefix).
• test_session_delete_file — top-level delete, nested delete, idempotent no-op on missing.
• test_session_read_range — mid-file range, start-of-file, up-to-end, zero-length read, and OSError on a missing object.
• All existing session tests pass.
• cargo fmt; cargo clippy -- -D warnings clean (default features).

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