Do not use as const with Z3 due to soundness issue

[tautschnig]

Disable as const for Z3 as it produces wrong results per Z3Prover/z3#9550.

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[Copilot]

Pull request overview

Disables the SMT2 as const array constant construct for the Z3 backend due to a soundness bug in Z3 (Z3Prover/z3#9550) that causes incorrect results.

Changes:

• Set use_as_const = false for the Z3 solver path in smt2_convt's constructor.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.55%. Comparing base (82ba0cc) to head (e9816ba).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9011      +/-   ##
===========================================
- Coverage    80.55%   80.55%   -0.01%     
===========================================
  Files         1707     1707              
  Lines       189047   189047              
  Branches        73       73              
===========================================
- Hits        152292   152287       -5     
- Misses       36755    36760       +5     

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[rod-chapman]

mlkem-native test results using patched CBMC (and standard Z3 4.15.3) look good. No regressions. A bit slower at K=4, but not too bad. A bit faster for K=2 and K=3.

[rod-chapman]

mldsa-native test results using patched CBMC (and standard Z3 4.15.3) look good. No regressions. A bit faster too for all parameter sets.