Add string type support to SMT back-end

[tautschnig]

We can safely thread through string operations to the back-end as SMT solvers now have support for the theory of strings.

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• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
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• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[Copilot]

Pull request overview

This pull request adds basic string type support to the SMT2 backend solver, enabling SMT solvers to handle string operations using the SMT-LIB theory of strings.

Changes:

• Added support for converting string types to SMT2 "String" type
• Implemented string constant conversion with escaping for double quotes and backslashes
• Added special handling for Str.Concat function application to convert to SMT-LIB str.++ operator

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[Copilot]

The str.++ (string concatenation) operation in SMT-LIB 2 can take any number of arguments (including 0, 1, or more than 2), but this implementation only handles exactly 2 arguments. When there are != 2 arguments, it falls through to the else case which will output "Str.Concat" instead of "str.++". Consider removing the size check or handling variable numbers of arguments to ensure all Str.Concat calls are properly converted to str.++.

Suggested change

	if(fn_name == "Str.Concat" &&
	function_application_expr.arguments().size() == 2)
	{
	out << "(str.++ ";
	convert_expr(function_application_expr.arguments()[0]);
	out << ' ';
	convert_expr(function_application_expr.arguments()[1]);
	if(fn_name == "Str.Concat")
	{
	out << "(str.++";
	for(const auto &op : function_application_expr.arguments())
	{
	out << ' ';
	convert_expr(op);
	}

[Copilot]

Inconsistent spacing around the == operator. The codebase convention in this file is to use no spaces (e.g., type.id()==ID_string on line 6025, expr_type.id()==ID_string on line 3782). This line has spaces around == which is inconsistent with the rest of the file.

[codecov]

Codecov Report

❌ Patch coverage is 27.27273% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.00%. Comparing base (15eb10a) to head (84732a2).

Files with missing lines	Patch %	Lines
src/solvers/smt2/smt2_conv.cpp	27.27%	16 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8838      +/-   ##
===========================================
- Coverage    80.00%   80.00%   -0.01%     
===========================================
  Files         1700     1700              
  Lines       188252   188273      +21     
  Branches        73       73              
===========================================
+ Hits        150613   150621       +8     
- Misses       37639    37652      +13     

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