Add Ruby 4.0 preview to test matrix

[Copilot]

Adds ruby-head (Ruby 4.0 preview) to the GitHub Actions test matrix to detect compatibility issues early.

Changes

• Added matrix entry for ruby-head with Rails 8.0, PostgreSQL, and Active Storage
• Marked as experimental: true with continue-on-error to prevent blocking CI on preview failures

The ruby-head tests run alongside existing Ruby 3.2-3.4 configurations but won't fail the build, allowing proactive monitoring of Ruby 4.0 compatibility without disrupting development.

Original prompt

Add Ruby 4.0 preview to test matrix

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.21%. Comparing base (ab64a02) to head (b73b567).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3531   +/-   ##
=======================================
  Coverage   97.21%   97.21%           
=======================================
  Files         288      288           
  Lines        7608     7608           
=======================================
  Hits         7396     7396           
  Misses        212      212           

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