Fix segfault in k-induction step case with nested loops

[tautschnig]

The original code assumed loop_head->condition() always contained the loop guard, but in nested loop scenarios, the loop structure can vary. The backedge might contain the condition instead, or the loop might be unconditional.

Also, when processing nested loops, modifying the outer loop's goto-program could invalidate iterators pointing to the inner loop, causing memory access violations.

Co-authored-by: Kiro autonomous agent

Fixes: #5357

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[Copilot]

Pull request overview

Fixes a segmentation fault in k-induction instrumentation when handling nested loops by making loop-guard detection more robust and avoiding iterator invalidation during loop processing.

Changes:

• Added find_loop_guard to derive the loop guard from either the backedge or the loop-head exit goto (and handle unconditional loops).
• Changed loop traversal to only instrument outermost loops to avoid invalidating nested-loop iterators.
• Added a regression test covering nested loops.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/goto-instrument/k_induction.cpp	Robustly locate loop guards and avoid processing nested loops directly to prevent segfaults
regression/k-induction/nested-loops/test.desc	New regression test expectations for nested-loop k-induction instrumentation
regression/k-induction/nested-loops/main.c	New nested-loop reproducer for the prior segfault

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[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.68%. Comparing base (02c2769) to head (5f87be7).

Files with missing lines	Patch %	Lines
src/goto-instrument/k_induction.cpp	90.90%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8803      +/-   ##
===========================================
- Coverage    80.69%   80.68%   -0.01%     
===========================================
  Files         1714     1714              
  Lines       189597   189612      +15     
  Branches        73       73              
===========================================
+ Hits        152987   152997      +10     
- Misses       36610    36615       +5     

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