Synchronize distributed remote downloads

[fallintoplace]

Summary

• synchronize remote download_file() calls across distributed ranks before returning the resolved path
• propagate rank-0 download failures to every rank instead of letting non-rank0 proceed with a stale or missing path
• validate that the final downloaded file exists after synchronization and add a hermetic regression test for the success, failure, and missing-file cases

Why

download_file() only fetched on rank 0, but the other ranks returned the target path immediately. In multi-GPU inference that lets non-rank0 code race ahead and try to open the file before rank 0 finishes wiring up the symlink and metadata.

I used broadcast_object_list() instead of a plain barrier so the same rendezvous also carries any rank-0 download error to the other ranks, which avoids turning an early rank-0 failure into a later hang or a confusing per-rank file error.

Validation

• uvx ruff check /Users/hoangvu/Code/OSS/cosmos-framework/cosmos_framework/inference/common/args.py /Users/hoangvu/Code/OSS/cosmos-framework/cosmos_framework/inference/common/download_file_sync_test.py
• uvx ruff format --check /Users/hoangvu/Code/OSS/cosmos-framework/cosmos_framework/inference/common/args.py /Users/hoangvu/Code/OSS/cosmos-framework/cosmos_framework/inference/common/download_file_sync_test.py
• PYTHONPATH=/Users/hoangvu/Code/OSS/cosmos-framework uvx pytest --noconftest /Users/hoangvu/Code/OSS/cosmos-framework/cosmos_framework/inference/common/download_file_sync_test.py -o addopts=