test(e2e): wait warmup slots in slashing tests

[spalladino]

Attempt at deflaking slashing tests by keeping some warmup slots. Codex analysis below.

Cause

The duplicate proposal slash test was timing-fragile after proposer pipelining. In the failed CI run, the malicious shared-key validators were selected for the first slot of the target epoch immediately after the test started sequencers and warped time forward.

Because pipelining builds a proposal one slot ahead, selecting the first slot meant the malicious nodes had to start building at the exact warp boundary. They started late in the build sub-slot, serialized the duplicate proposals after the receiver nodes had already advanced to the next slot, and the receivers rejected the gossip as late (invalid slot number) before duplicate-proposal detection could run.

Failed run: 36837b7edc543e70

Analysis

The failed run never produced a DUPLICATE_PROPOSAL offense. It only produced a DUPLICATE_ATTESTATION offense, then timed out waiting for the expected proposal offense.

Timeline from the failed run:

• malicious validators were selected for slot 10
• both started building late in sub-slot 3
• both broadcast checkpoint proposals for slot 10
• receivers had already advanced to slot 11 and rejected slot 10 proposals as stale
• the generic offense wait was satisfied by a duplicate attestation
• the final DUPLICATE_PROPOSAL assertion timed out

A passing run selected a later proposer slot instead. That gave the sequencers a full warmup slot after the warp, both malicious nodes built early enough, standalone duplicate proposals were broadcast in time, and DUPLICATE_PROPOSAL was detected.

Fix Rationale

Update advanceToEpochBeforeProposer to skip the first warmupSlots slots of the target epoch and return the concrete targetSlot. The duplicate proposal and duplicate attestation tests still warp to one slot before the target epoch, but now the selected malicious proposer slot is at least one slot into that epoch.

That gives freshly-started sequencers one full slot of wall-clock warmup before the pipelined build for the malicious slot begins, avoiding the startup/warp boundary race where duplicate proposals are emitted too late and rejected before slash detection.