Print the actual Expr for an add_requirement() that fails.

[mcourteaux]

Cherry picked from #9337.

A requirement that fails got simplified to false before being turned into a string.
This PR stringifies the Expr upon registering the requirement. This is way more meaningful than reading false at all times.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.03%. Comparing base (6e5de35) to head (0e1f87a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9424      +/-   ##
==========================================
+ Coverage   69.93%   70.03%   +0.10%     
==========================================
  Files         261      261              
  Lines       79410    79412       +2     
  Branches    19363    19363              
==========================================
+ Hits        55532    55618      +86     
+ Misses      17937    17916      -21     
+ Partials     5941     5878      -63     

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