Migrate fetch_artifacts.py to use artifact_backend and artifact_manager

[ScottTodd]

Motivation

While working on #3000, I found that this code is directly handling S3 paths like so:

    def __post_init__(self):
        self.s3_key_path = f"{self.external_repo}{self.workflow_run_id}-{self.platform}"

I'm trying to move that code to a central location and to make it possible to swap out the "s3 cloud backend for a "local backend" for easier testing. Moving string manipulation code is easy enough, but this file also directly communicated with S3:

          log(f"++ Downloading {artifact_key} to {output_path}")
          with open(output_path, "wb") as f:
              s3_client.download_fileobj(bucket, artifact_key, f)

That would be harder to refactor... if we didn't already have this abstraction in ArtifactBackend. So, I'm first migrating this code to use that class.

Technical Details

I also considered making fetch_artifacts.py just call artifact_manager.py directly, but they have different roles:

• artifact_manager.py is topology-aware and is currently just used for multi-arch builds
• fetch_artifacts.py has regex-based fetching and is currently used by developers to fetch explicit lists of artifacts and by CI workflows to download subsets of artifacts for tests

Test Plan

• Existing unit tests (light coverage)
• Manual sanity check:

  python ./build_tools/fetch_artifacts.py ^
    --run-id=18480879899 ^
    --artifact-group=gfx110X-dgpu ^
    --output-dir=%HOME%\.therock\18480879899\artifacts

• CI test jobs will run this fetch_artifacts.py script indirectly via build_tools/install_rocm_from_artifacts.py

Test Result

• CI jobs run the script with no issues, e.g. https://github.com/ROCm/TheRock/actions/runs/21192582915/job/60969434096?pr=3019#step:6:280

  Calling fetch_artifacts_main with args:
    --run-id 21192582915 --artifact-group gfx94X-dcgpu --output-dir build --flatten core-hipinfo_run core-runtime_run core-runtime_lib sysdeps_lib base_run base_lib amd-llvm_run amd-llvm_lib core-hip_lib core-hip_dev core-ocl_lib core-ocl_dev rocprofiler-sdk_lib host-suite-sparse_lib blas_lib blas_test
  
  ...
  
  ++ Flattening 'blas_test_gfx94X-dcgpu.tar.xz' to 'blas_test_gfx94X-dcgpu'
  ++ Flattening 'blas_lib_gfx94X-dcgpu.tar.xz' to 'blas_lib_gfx94X-dcgpu'
  Retrieved artifacts for run ID 21192582915
  

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.

[ScottTodd]

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