Adopt head_v2 event semantics across head-event consumers #9659

PR #9486 introduces the head_v2 event and keeps the existing head event for compatibility. During review, Nico suggested that the new head_v2 semantics are nicer and may be worth adopting more broadly instead of continuing to pass around older head event data shapes.

This issue tracks the follow-up refactor outside of #9486's implementation scope.

Tasks:

• Audit current consumers/producers of the existing head event data shape.
• Decide whether Lodestar should replace internal head event usage with head_v2 semantics, or keep head_v2 as the canonical internal shape and add a compatibility shim for the legacy v1 head event.
• Update validator-side and beacon-node-side callers consistently once feat: implement head_v2 event #9486 lands.
• Preserve backwards compatibility for external consumers of the existing head event.
• Add/adjust tests around v1/v2 event payload compatibility and revalidation behavior.
• Re-subscribe the validator client from head to head_v2 once head_v2 is broadly supported across clients. Tagged for gloas for now; revisit as head_v2 adoption progresses across clients toward mainnet, and consider enabling after the gloas fork (per feat: implement head_v2 event #9486 discussion with @nflaig).

Origin/dependency:

• Originating review discussion: feat: implement head_v2 event #9486 (comment)
• Depends on / follows from PR feat: implement head_v2 event #9486: feat: implement head_v2 event #9486