Embedded IAM and STS routes now report authentication, form-parse and
authorization failures with the IAM ErrorResponse body instead of the
S3 Error envelope, so IAM SDK clients can parse them. Requests signed
for s3 keep the S3 envelope, keyed off the credential scope.
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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>