[tautschnig]

The integer-to-fixedbv conversion in the SMT2 back end checked
dest_type.id() == ID_unsignedbv to decide between zero_extend and
sign_extend. Since dest_type is always fixedbv in this branch, the
condition was always false, and sign_extend was used unconditionally.
This caused unsigned integers (and unsigned enums) to be sign-extended
when converted to fixedbv, producing wrong negative values.

Fix: check src_type instead, also testing the underlying type for
c_enum sources. Also fix the missing opening parenthesis in the
zero_extend branch, which produced malformed SMT-LIB.

Fixes #9213

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.91%. Comparing base (266f438) to head (af2fed3).

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	0.00%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9232      +/-   ##
===========================================
- Coverage    80.91%   80.91%   -0.01%     
===========================================
  Files         1718     1718              
  Lines       190266   190268       +2     
  Branches        73       73              
===========================================
- Hits        153952   153949       -3     
- Misses       36314    36319       +5     

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