test(test-execute): pin genesis commitment provider

[chugarchugarr]

Description

Addresses the remaining regression-coverage gap in #3251. The production MPT decoupling has already landed in #3279; this PR pins both execute-path boundaries that issue identified.

The existing execute/fill parity test could agree on the wrong root because it reused the allocation whose private commitment provider had already been selected by build_genesis_header().

This adds independent assertions for both paths:

1. Hive execute builder: serialize and reconstruct the logical allocation, independently select the genesis fork's provider, compute the expected root, and compare it with the execute genesis header.
2. eth_config genesis model: construct a non-empty BinaryTree genesis allocation, independently compute its MPT contrast root, and prove model initialization selects PBT before the genesis hash is evaluated.

Genesis-header parity is therefore no longer sufficient by itself; each execute boundary must independently agree with the fork-selected commitment provider.

Verification

• uv run pytest -q packages/testing/src/execution_testing/cli/tests/test_execute_genesis.py packages/testing/src/execution_testing/cli/pytest_commands/plugins/execute/eth_config/tests/test_genesis.py — 42 passed
• Ruff check and format check — passed
• git diff --check — passed

Scope

• test-only
• no duplicate state-commitment plumbing
• no MPT→PBT activation/migration machinery
• no mainnet fork behavior change

Base

projects/binary-trie at 09d2088cf9f338e660cb04f948653baf544b7ba6.