consume direct: block-test failures against Nethermind nethtest are never detected #3609

consume direct --bin=nethtest cannot report a failing blockchain_test against Nethermind.

Two facts combine:

1. nethtest --blockTest always exits 0 and reports results only as human-readable <name> PASS/FAIL lines on stdout (src/Nethermind/Nethermind.Test.Runner/Program.cs, Run returns 0 unconditionally; BlockchainTestsRunner.cs prints per-test lines, no JSON summary). State tests do emit a JSON array, block tests do not.
2. packages/testing/src/execution_testing/client_clis/clis/nethermind.py decides the block-test outcome from result.returncode alone and never inspects stdout.

Reproduce: take any blockchain_test fixture, change a surviving storage value in its pre so the executed root no longer matches the RLP-committed root, run uv run consume direct --input <dir> --bin=nethtest -m blockchain_test. Nethermind itself prints FAIL (or throws InvalidBlockException), the harness reports the test as passed.

Verified with Nethermind built from current master (dotnet build of Nethermind.Test.Runner).

Found while working on #3600