fix(core): harden vercel deploy workflows

[harishsundar-okta]

Summary

Hardens the Vercel deployment workflows against supply chain attacks and credential information disclosure.

Why

A security review identified three vulnerabilities in the production deployment workflow:

• Supply chain risk — vercel@latest was used, allowing a compromised future version to execute in CI.
• Information disclosure — An unnecessary echo statement logged deployment context to the terminal, risking credential exposure.
• Information disclosure — The Vercel API token was passed via ${{ secrets.VERCEL_TOKEN }} inline string interpolation in a bash script, which can leak the value in logs if the GitHub auto-scrubbing fails to detect it.

What

• production-deploy.yml: Pinned Vercel CLI from vercel@latest to vercel@39.3.0.
• production-deploy.yml: Removed the echo "Deploying to Vercel (Production)..." line.
• production-deploy.yml: Moved VERCEL_TOKEN to a step-level env variable instead of using ${{ secrets.VERCEL_TOKEN }} inline in the run script.
• preview-deploy.yml: Quoted $VERCEL_TOKEN and $VERCEL_ORG_ID variable references in the deploy command to prevent word-splitting and accidental exposure.

Testing

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Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.64%. Comparing base (1f2c1e5) to head (0d50c66).
⚠️ Report is 49 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #240   +/-   ##
=======================================
  Coverage   88.64%   88.64%           
=======================================
  Files         154      154           
  Lines       13127    13127           
  Branches     1711     1711           
=======================================
  Hits        11637    11637           
  Misses       1490     1490           

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