Fix builtin function return hidden flag inconsistency

[tautschnig]

Function returns for built-in functions now correctly match their function call hidden flag. This is done by symex storing the combined hidden flag in call frame.

Co-authored-by: Kiro autonomous agent

Fixes: #5207

• Each commit message has a non-empty body, explaining why the change was made.
• 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• 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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.01%. Comparing base (b191cc1) to head (5145ed9).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8800   +/-   ##
========================================
  Coverage    80.01%   80.01%           
========================================
  Files         1700     1700           
  Lines       188345   188345           
  Branches        73       73           
========================================
+ Hits        150696   150697    +1     
+ Misses       37649    37648    -1     

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[Copilot]

Pull request overview

Fixes an inconsistency where built-in function return events could have a different hidden flag than their corresponding function call, by storing the combined hidden flag on the call frame and adding regression coverage for JSON/XML UIs.

Changes:

• Store the combined hidden value on the call frame for later use by return handling.
• Add a regression test that asserts hidden="false" / "hidden": false for both function-call and function-return steps for malloc in XML and JSON UIs.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/goto-symex/symex_function_call.cpp	Persists the combined hidden flag on the call frame to keep call/return hidden status consistent.
regression/cbmc/builtin-function-hidden-flag/test.c	Adds a minimal program that triggers a built-in (malloc) call and a failing assertion to produce a trace.
regression/cbmc/builtin-function-hidden-flag/test.desc	Adds JSON-UI regression expectations for hidden: false on call and return steps.
regression/cbmc/builtin-function-hidden-flag/test_xml.desc	Adds XML-UI regression expectations for hidden="false" on call and return steps.

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