fix(ef-tests): report stateless validation failures

[Osraka]

Summary

Report stateless validation failures from the EF test runner as Error::BlockProcessingFailed instead of panicking via expect.

This keeps the failing block number and partial program inputs attached to the error, which makes fixture debugging easier without changing execution or validation semantics.

Related to #18.

Changes

• Move the stateless validation step into a small helper.
• Map signer recovery and stateless validation failures into the existing test error type.
• Preserve partial program inputs up to the failing block.
• Add a regression test covering an empty-witness validation failure.

Tests

• cargo test -p ef-tests -- --nocapture
• cargo fmt --all --check
• git diff --check

cargo clippy -p ef-tests --all-targets -- -D warnings was not run locally because the stable toolchain does not have the clippy component installed.