Reject GCS blob names that escape the target directory on download (#67509)

``GCSHook.sync_to_local_dir`` and ``GCSTimeSpanFileTransformOperator._download``
joined GCS blob names into local paths without verifying the resolved
path stayed within the intended directory. GCS allows object names
containing ``..`` segments, so a hostile blob name could cause files
to be written outside ``local_dir`` / the operator's temp dir — a
classic CWE-22 path-traversal sink. The trust model matters: a DAG
author's own bucket is trusted, but operators are routinely pointed
at buckets shared with external partners or other tenants, where the
write side may not be fully trusted.

Resolve the destination and assert ``is_relative_to`` the target root
before any download. On violation, raise ``ValueError`` with a clear
message instead of silently writing outside the target.