Improve observability for sync committee subnets

[akronim26]

PR Description

This PR adds observability around sync committee peering, making it easy to see when a subnet is under-peered. It adds:

• SyncCommitteeSubnetPeerCountLogger, which issues throttled warnings whenever the peer count is less than the target count
• sync_committee_message_peer_search_requested_total is labelled with reason (NO_PEERS or NO_OUTBOUND_STREAM) so the two cases can be observed and handled accordingly

Fixed Issue(s)

Resolves #11114

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

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

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Note

Low Risk
Observability-only changes (logging and metrics) with no changes to gossip publish or peer-selection behavior beyond instrumentation.

Overview
Adds observability for sync committee subnet peering so operators can spot under-peered subnets and distinguish why gossip publish failures trigger peer discovery.

Introduces SyncCommitteeSubnetPeerCountLogger, invoked whenever PeerSubnetSubscriptions is rebuilt during peer selection. For each subscribed sync committee subnet with at least one connected peer, it emits throttled (60s per subnet) warnings if the subscribed peer count is below the configured target.

In SyncCommitteeMessageGossipManager, peer searches on publish failure are now counted via sync_committee_message_peer_search_requested_total with a reason label (no_peers vs no_outbound_stream), without changing when peer search runs. Network builders and test fixtures pass the new logger into PeerSubnetSubscriptions.create.

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[rolfyone]

One non-blocking note: GitHub Advanced Security already commented on the two unused switch pattern variables in SyncCommitteeMessageGossipManager.java:140-141. Replacing ignored with unnamed patterns or reverting to instanceof checks would quiet that.

[akronim26]

One non-blocking note: GitHub Advanced Security already commented on the two unused switch pattern variables in SyncCommitteeMessageGossipManager.java:140-141. Replacing ignored with unnamed patterns or reverting to instanceof checks would quiet that.

I have replaced the unread pattern with instanceof checks