ci: diagnose macOS figshare 403 errors

[sethaxen]

Summary

Since the v0.4.3 artifact update, all 8 new datasets fail to download on macOS CI with HTTP 403 from figshare (awselb/2.0). Old datasets work because they are cached.

This PR adds diagnostics to identify the root cause:

• aarch64 macOS matrix entry: tests whether running native ARM64 Julia (instead of x64 under Rosetta) avoids the issue
• curl test: checks if figshare blocks the runner IP at the network level
• Downloads.jl vs HTTP.jl: checks if the issue is specific to HTTP.jl's request handling

Hypotheses

1. Figshare blocks macOS runner IPs (different subnet than Linux/Windows)
2. HTTP.jl sends headers/TLS fingerprint that figshare rejects on macOS
3. Rosetta emulation (x64 on ARM64) causes the issue

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.31%. Comparing base (1f9f17d) to head (24f5d1a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #46   +/-   ##
=======================================
  Coverage   84.31%   84.31%           
=======================================
  Files           5        5           
  Lines          51       51           
=======================================
  Hits           43       43           
  Misses          8        8           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[sethaxen]

The issue was FigShare blocking some of GitHub's IP addresses; these have now been whitelisted.