test: cover geth fixture cleanup on errors

[vuyua9]

What was wrong?

start_geth_process_and_yield_port now uses a finally block to stop the geth process when a test fails after the fixture yields, but that cleanup path was not covered by a focused regression test.

Related to the go-ethereum integration test fixture cleanup path.

How was it fixed?

Added a small test that drives the fixture generator directly with monkeypatched geth process helpers. The test throws a consumer exception into the generator and asserts that:

• kill_proc_gracefully() is called
• proc.communicate(timeout=5) is called
• the process is killed before communicate() is invoked

Todo:

• Clean up commit history
• Add or update documentation related to these changes: not needed, test-only regression coverage
• Add entry to the release notes: added newsfragments/3851.internal.rst

Tests

• git diff --check
• .venv/bin/python -m pytest tests/integration/go_ethereum/test_conftest_cleanup.py -q

Note: uv pip install -e '.[test]' hit a local safe-pysha3 build issue in this macOS CommandLineTools environment, so I installed the package plus the focused dependencies needed for this test and ran the targeted pytest above.

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