fix missing attestations by using GossipSub for subnet peer coverage (#19523)

## Summary
- Fix intermittent missing attestations caused by inaccurate subnet peer
coverage calculation. The previous ENR-based approach counted peers that
*claimed* to support subnets, while GossipSub mesh often had far fewer
actual subscribers (e.g., ENR showed 3 peers but GossipSub only had 1).
- Add proactive subnet peer search that runs every slot (12s) to find
and connect peers for underserved subnets (< 4 peers).
- Add subnet-aware peer pruning that avoids disconnecting peers covering
critical subnets.
- Allow subnet-useful peers past the peer limit in `onConnection` so
proactive search results are not immediately rejected.

## Root Cause
Attestations were published with only 1-2 peers on the GossipSub topic,
causing them to not propagate. The Sentinel used ENR `attnets` to
estimate subnet coverage, but ENR represents *capability* (what a peer
claims to support), not *actual GossipSub subscriptions*. This led to
the system believing it had sufficient peers when it didn't, skipping
peer search for those subnets.

## Changes
- **`cl/sentinel/discovery.go`**: Replace ENR-based
`getSubnetCoverage()` with GossipSub `ListPeers()` for accurate
coverage; add `proactiveSubnetPeerSearch()`, `findPeersForSubnets()`,
`pruneExcessPeers()`; subnet-aware `onConnection` handler
- **`cl/sentinel/sentinel.go`**: Store `*enode.Node` in `pidToEnr` (was
`string`) for ENR data access in `onConnection`
- **`cl/phase1/network/gossip/gossip_manager.go`**: Log peer count when
publishing attestations; remove `MinTopicSize(1)` to avoid publish
failures on empty topics
- **`cl/beacon/handler/block_production.go`**: Add subnet info to
"Produced Attestation" debug log

https://beaconcha.in/validator/1667009#charts

<img width="937" height="636" alt="截圖 2026-02-27 晚上7 48 54"
src="https://github.com/user-attachments/assets/e435b9ac-3ee3-444f-a4c1-fb1891d381b7"
/>

Author: domiwei

cl/beacon/handler/block_production.go

@@ -47,6 +47,7 @@ import (
 	"github.com/erigontech/erigon/cl/gossip"
 	"github.com/erigontech/erigon/cl/persistence/beacon_indicies"
 	"github.com/erigontech/erigon/cl/phase1/core/state"
+	"github.com/erigontech/erigon/cl/phase1/network/subnets"
 	"github.com/erigontech/erigon/cl/transition"
 	"github.com/erigontech/erigon/cl/transition/impl/eth2"
 	"github.com/erigontech/erigon/cl/transition/machine"
@@ -143,8 +144,13 @@ func (a *ApiHandler) GetEthV1ValidatorAttestationData(
 	}
 
 	defer func() {
+		epoch := *slot / a.beaconChainCfg.SlotsPerEpoch
+		committeesPerSlot := a.syncedData.CommitteeCount(epoch)
+		subnet := subnets.ComputeSubnetForAttestation(
+			committeesPerSlot, *slot, *committeeIndex,
+			a.beaconChainCfg.SlotsPerEpoch, 64)
 		a.logger.Debug("Produced Attestation", "slot", *slot,
-			"committee_index", *committeeIndex, "cached", ok, "beacon_block_root",
+			"committee_index", *committeeIndex, "subnet", subnet, "cached", ok, "beacon_block_root",
 			attestationData.BeaconBlockRoot, "duration", time.Since(start))
 	}()
 

cl/p2p/mock_services/p2p_manager_mock.go

@@ -261,8 +261,18 @@ func (g *GossipManager) Publish(ctx context.Context, name string, data []byte) e
 	if topicHandle == nil {
 		return fmt.Errorf("topic not found: %s", topic)
 	}
+	// Log peer count for attestation topics to help diagnose propagation issues
+	if gossip.IsTopicBeaconAttestation(name) {
+		peerCount := len(g.p2p.Pubsub().ListPeers(topic))
+		if peerCount == 0 {
+			log.Warn("[Gossip] Publishing attestation with NO peers on subnet", "topic", name, "peerCount", peerCount)
+		} else if peerCount < 3 {
+			log.Debug("[Gossip] Publishing attestation with low peer count", "topic", name, "peerCount", peerCount)
+		}
+	}
 	// Note: before publishing the message to the network, Publish() internally runs the validator function.
-	return topicHandle.topic.Publish(ctx, compressedData, pubsub.WithReadiness(pubsub.MinTopicSize(1)))
+	// Removed MinTopicSize(1) - don't fail if no peers on subnet, message will propagate when peers join
+	return topicHandle.topic.Publish(ctx, compressedData)
 }
 
 func (g *GossipManager) goCheckForkAndResubscribe(ctx context.Context) {