Stop emitting duplicate head events for gloas

[Matilda-Clerke]

PR Description

Stop emitting head v1 events post-GLOAS

Fixed Issue(s)

#10976

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

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Note

Medium Risk
Changes observable REST event behavior for integrations subscribed to head on GLOAS networks; mitigated because head_v2 continues to fire for the same updates.

Overview
On GLOAS and later milestones, beacon REST SSE subscribers no longer receive duplicate head (v1) events when the chain head updates; head_v2 remains emitted for every head update as before.

EventSubscriptionManager.chainHeadUpdated now only calls notifySubscribersOfEvent(EventType.head, …) when spec.atSlot(slot).getMilestone() is strictly before SpecMilestone.GLOAS. Pre-GLOAS behavior is unchanged.

Tests were updated for a minimal Gloas spec to assert event: head is absent while head_v2 is still delivered when clients subscribe to both topics. The changelog records this under bug fixes.

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Head stop breaks remote validators

High Severity

Gating head behind pre-GLOAS stops events that Teku's remote validator still exclusively consumes. That client only subscribes to head and uses it for onHeadUpdate duty revalidation and early attestation triggers, so after GLOAS those validators receive no head notifications and duties may not recalculate on reorgs.

 

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[Matilda-Clerke]

This is addressed in #11130

[rolfyone]

LGTM