test: add Rex4 system contract deployment and idempotence tests -v2

[viktorcrypt]

Summary

Adds Rex4 deployment tests for MegaAccessControl and MegaLimitControl, covering:

• activation on Rex4
• idempotent repeated apply_pre_execution_changes()
• Rex3 boundary behavior (not deployed before Rex4)

Also incorporates the requested review follow-up:

• use production-like hardfork activation with with_all_activated()
• check KECCAK_EMPTY for the non-deployed case
• extract repeated executor setup into a shared helper

Note

This PR replaces #187, which got stuck due to a GitHub fork-lineage issue after the repository outage. All review feedback from Troublor has been incorporated here. Please consider closing #187 in favor of this one.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.5%. Comparing base (2bba707) to head (ee21de5).
⚠️ Report is 13 commits behind head on main.

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[Troublor]

LGTM