Clang 21 migration

[iartemov-ledger]

• Fixing LEDGER_ASSERT failures with strings exceeding internal MESSAGE_SIZE hard-coded to 50
• Removing unused key_expression_index variable
• Fixing count_m variable use
  • A non-regression UT

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.81%. Comparing base (1fdcbd7) to head (e20c3a5).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #427   +/-   ##
========================================
  Coverage    85.81%   85.81%           
========================================
  Files           18       18           
  Lines         2812     2813    +1     
  Branches       425      426    +1     
========================================
+ Hits          2413     2414    +1     
  Misses         388      388           
  Partials        11       11           

Flag	Coverage Δ
unittests	85.81% <100.00%> (+<0.01%)	⬆️

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[bigspider]

LGTM