Add tests with more than a single shard

Currently all tests (including e2e tests) only have one shard in the main core proof, leading to some verifier codepaths and conditional constraints/checks not being exercised by CI

Ideally, this test should exercise both the rust and the recursive verifiers

Start by locating the main core proof test and its current single-shard configuration, then trace how the Rust and recursive verifiers are selected. Run the existing proof and e2e tests before changing coverage; done means CI exercises proofs with more than one shard through both verifier paths and reaches the relevant conditional checks.

Written by the indexing model from the issue text.