fix(async-upload): scope S3 downloads to directory prefix

[immanuwell]

Description

S3 treats Prefix as a raw string. With my-model, a bucket containing my-model/model.onnx and my-model-backup/unrelated.bin returns both
The second path lands outside the staging dir, pretty easy to hit in shared buckets

This adds the trailing / for directory listings. Keys already ending in / stay unchanged.

How Has This Been Tested?

Repro:

1. Add the two keys above to MinIO.
2. Run the job with MODEL_SYNC_SOURCE_AWS_KEY=my-model.
3. Before this fix both objects are downloaded, now only my-model/model.onnx is.

Passed all 55 async upload unit tests, make build/compile, make lint, and make vet
A real MinIO check passed too

Broad make test was also run, but unrelated MySQL testcontainers timed out on startup

Merge criteria:

• All commits are signed off.
• The commit has a meaningful message.
• Automated tests are included.
• The change was manually tested with MinIO.
• Code follows the contribution guidelines.

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[pboyd]

Thanks for the PR, @immanuwell.

One question though, what happens if config.key were my-model/model.onnx? I believe that would work today, but it would break after this change because it would become my-model/model.onnx/.