Prevent the macOS test workflow from inheriting TNG_AUTH_AGENT, which can
redirect authentication tests to the Spindle agent instead of the intended
test configuration.
Also make release artifact publishing resilient to transient upload failures:
- retry an absent artifact up to three total attempts with bounded backoff, including transient rate-limit responses during upload or inspection;
- inspect the repository after each failed upload;
- accept an already-published artifact only when its SHA-256 matches;
- stop without replacement when inspection fails or an existing artifact differs.
The workflow YAML and embedded shell were validated locally, and the complete Swift test suite passes (626 tests in 82 suites).