test(e2e): wait for checkpoint before sentinel assertions in e2e_p2p_multiple_validators_sentinel

[spalladino]

Summary

• Stabilizes the multiple-validator sentinel e2e by waiting for a post-warmup checkpoint before recording the assertion window.
• Reuses the same warm-up helper in the second test so isolated runs avoid the same fresh-network startup noise before stopping a validator.

Failed run

Failed CI run: http://ci.aztec-labs.com/07fb31bc0706159f

The failing test was e2e_p2p_multiple_validators_sentinel > collects attestations for all validators on a node. The test expected no attestation-missed entries, but the assertion window started while the network was still in the first pipelined slots after startup. In the failed run, slot 8 was built on a pending, not-yet-checkpointed parent, so some remote validators could not validate/attest in time and the sentinel recorded a missed attestation.

Fix

The test now waits for one warm-up slot and then waits for the observed checkpoint number to advance before capturing initialSlot. That keeps startup pipelining behavior out of the strict sentinel assertion window while preserving the test's actual coverage: once the network is past warm-up, every validator should be observed attesting or proposing as expected.

Verification

• yarn format end-to-end
• yarn build
• yarn workspace @aztec/end-to-end test:e2e e2e_p2p/multiple_validators_sentinel.parallel.test.ts -t 'collects attestations for all validators on a node'

[spalladino]

Grind run green on http://ci.aztec-labs.com/1779889164003366