Add fork choice store pruning test

[Itodo-S]

Summary

Add a fork choice filler for store pruning on finalization.

Closes #567

What changed

• Add a devnet fork choice test covering pruning after finalization advances to slot 3
• Verify stale attestation data targeting slots 1, 2, and 3 is removed
• Verify attestation data targeting slots 4 and 5 is preserved
• Extend StoreChecks with exact target-slot assertions for raw, new, and known attestation pools
• Update fork choice attestation steps to retain raw gossip signatures so pruning can be tested end to end

Why

Pruning stale attestation data is required to avoid unbounded memory growth and to ensure finalized targets no longer influence fork choice state.

[tcoratger]

@Itodo-S Looks like test vectors are failing, can you check?

[Itodo-S]

@tcoratger, I have worked on this and pushed already; that should be resolved now.