Upgrade spinner token comparison logic for hardened constant-time security

[dmrAnderson]

• Replaced standard string comparison with a secure, constant-time comparison to prevent potential timing attacks.
  While the practical risk here is low, upgrading to a constant-time comparison is a zero-cost fix that aligns with standard Rails security conventions.
• Fix flaky timestamp-threshold specs. Relied on real elapsed time staying under the 1s threshold.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (b0093de) to head (f998d02).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
+ Coverage   98.44%   98.57%   +0.13%     
==========================================
  Files          22       22              
  Lines         449      492      +43     
==========================================
+ Hits          442      485      +43     
  Misses          7        7              

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