SMT2 back-end: flatten with_exprt operands

[tautschnig]

Solvers without datatypes support require flattening towards bitvectors. This is also true for array-typed expressions when the array is not at the top level (despite possible array-theory support). This changes makes sure we will flatten such array-typed expressions.

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[codecov]

Codecov Report

Attention: Patch coverage is 60.71429% with 11 lines in your changes missing coverage. Please review.

Project coverage is 80.39%. Comparing base (062962c) to head (9a1df4b).

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	60.71%	11 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8670   +/-   ##
========================================
  Coverage    80.39%   80.39%           
========================================
  Files         1688     1688           
  Lines       207383   207390    +7     
  Branches        73       73           
========================================
+ Hits        166731   166737    +6     
- Misses       40652    40653    +1     

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[tautschnig]

Note to reviewers: #8637 includes a test set-up with tests passing once the changes in this PR are applied. I am working on extracting a minified variant thereof to include as regression test. Edit: done.

[rod-chapman]

This branch does seems to fix the array flattening issue on the mlda-native code when using bitwuzla.

TWO functions still fail with a different error though. For example, the unpack-hints function in mldsa-native:

cbmc --bitwuzla --outfile b.smt2 gotos/unpack_hints_harness.goto
bitwuzla b.smt2

yields

[error] b.smt2:18036:529: expected terms of same sort at indices 1 and 2 as argument to 'ite'

Is this related, or a completely different problem?