feat(spec-specs, tests): add system address BAL tests

[flcl42]

🗒️ Description

Adds BAL coverage for SYSTEM_ADDRESS handling:

• test_bal_account_touch_system_address — parametrized across BALANCE, EXTCODESIZE, EXTCODEHASH, EXTCODECOPY, CALL, STATICCALL. Asserts that a real EVM touch from user code lands SYSTEM_ADDRESS in the BAL as an account-only entry.
• test_bal_invalid_surplus_system_address_from_system_call — asserts clients reject a BAL that includes SYSTEM_ADDRESS solely because it was the synthetic caller of a pre-execution system operation (EIP-4788 beacon roots call).

🔗 Related Issues or PRs

N/A.

✅ Checklist

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

  just 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 updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.26%. Comparing base (c3462e0) to head (cb724e7).
⚠️ Report is 2 commits behind head on forks/amsterdam.

Additional details and impacted files

@@               Coverage Diff                @@
##           forks/amsterdam    #2715   +/-   ##
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  Coverage            86.26%   86.26%           
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  Files                  599      599           
  Lines                37038    37038           
  Branches              3795     3795           
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  Hits                 31949    31949           
  Misses                4525     4525           
  Partials               564      564           

Flag	Coverage Δ
unittests	86.26% <ø> (ø)

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[spencer-tb]

LGTM! I added a few changes, would like to get it in the next release today :D
cc @fselmo for extra eyes, will merge now and anything else can be a follow up