[TEST - DO NOT MERGE] manifest-diff fork-PR coverage validation

[amd-hsivasun]

Test-only PR to validate the fork-PR composite-credentials path on manifest-diff.yml introduced by #4908.

This PR's content is identical to #4908; the only difference is that the head is on a fork (amd-hsivasun/TheRock) instead of in ROCm/TheRock, so we can verify:

1. The manifest_diff sibling job in multi_arch_ci.yml fires on a fork PR (i.e. the if: github.repository == 'ROCm/TheRock' gate passes because the workflow runs in the base repo's context).
2. The composite configure_aws_artifacts_credentials action takes the fork branch (skips OIDC, uses baseline creds mounted at /home/awsconfig/credentials.ini).
3. The report uploads to S3 and the link appears in the run's Step Summary.

Will be closed without merging once the run completes and the artifact link is captured.

[amd-hsivasun]

Closing as planned — this was a fork-PR coverage validation for #4908. manifest_diff job ran successfully on the fork-PR path (composite action took the baseline-creds branch, report uploaded to S3): https://github.com/ROCm/TheRock/actions/runs/25904194086/job/76134028768