Simplify fixed_opcode_count parameterization logic

[kamilchodola]

Refactor fixed_opcode_count handling to simplify logic and improve clarity.

🗒️ Description

🔗 Related Issues or PRs

N/A.

✅ Checklist

• All: Ran fast tox checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  uvx tox -e static

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered adding an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.
• Tests: For PRs implementing a missed test case, update the post-mortem document to add an entry the list.
• Ported Tests: All converted JSON/YML tests from ethereum/tests or tests/static have been assigned @ported_from marker.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.61%. Comparing base (9e161b6) to head (f2d2498).
⚠️ Report is 2 commits behind head on forks/amsterdam.

Additional details and impacted files

@@               Coverage Diff                @@
##           forks/amsterdam    #1925   +/-   ##
================================================
  Coverage            82.61%   82.61%           
================================================
  Files                  417      417           
  Lines                26862    26862           
  Branches              2492     2492           
================================================
  Hits                 22193    22193           
  Misses                4230     4230           
  Partials               439      439           

Flag	Coverage Δ
unittests	82.61% <ø> (ø)

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