Fixing path traversal/arbitrary out-of-directory file read via sharde…

[buffett0323]

What does this PR do?

Fixes a path-traversal vulnerability in sharded-checkpoint loading. Validate every shard filename right after parsing the index by rejecting any value where os.path.basename(f) != f. This can reject absolute paths, ../ traversal, and subdirectory components in one guard.

Fixes #14175

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