fix S3 presign URI for checkpoint archive deployment

[yu2001-s]

Summary

• pass the full s3://... URI to aws s3 presign when archiving checkpoint steps
• add focused unit coverage for the archive upload + presign command path
• verify presign failures still surface the AWS CLI stderr

Root cause

archive_and_presign_step_url() stripped the s3:// prefix before invoking aws s3 presign. The AWS CLI expects an S3 URI there, so Together deployment flows could fail after a successful archive upload because the presign command was handed an invalid path.

Validation

• uv run pytest tests/unit/test_s3_utils.py -q
• uv run ruff check src/art/utils/s3.py tests/unit/test_s3_utils.py
• uv run --with safetensors ty check src/art/utils/s3.py tests/unit/test_s3_utils.py

Notes

• uv run pytest --nbval --current-env --tb=short tests/unit -q is not runnable in this default local environment because several existing unit-test modules require optional heavy deps (torch, transformers, datasets) that are not installed.