feat(p2p): expose p2p connectivity (enabled + connected peer count)

[spalladino]

Adds getP2PConnectivity(), returning { enabled, connectedPeers }, so downstream subsystems can tell
whether the libp2p stack is alive and how many peers it is talking to. Until now a node running with a dead
p2p stack (zero peers) looked identical to a healthy one from the outside.

• P2PService declares the method synchronously, matching its neighbouring getPeers / getGossipMeshPeerCount.
• LibP2PService returns enabled: true and counts peers reported as connected by the peer manager, so
  dialing and cached peers are excluded.
• DummyP2PService (and the TXE dummy client) return enabled: false with zero peers, encoding "p2p is
  disabled" for sandbox and single-node setups; consumers can treat disabled p2p as vacuously healthy instead
  of as a stack with no peers.
• P2PClient forwards to the service, and P2PApi exposes it over JSON-RPC with a P2PConnectivity zod
  schema, so the node API surfaces it too.

No behavior is gated on this yet; it is the foundation for gating slashing, proposing, and sendTx on p2p
connectivity in follow-up changes.

Part of A-1701.

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Part of a stacked-PR chain (bottom → top) hardening the node against running with a dead p2p stack; each PR targets the branch below it and the bottom targets merge-train/spartan-v5:

1. fix(p2p): fail node startup when p2p service fails to start #25177 — fail node startup when the p2p service fails to start
2. feat(p2p): expose p2p connectivity (enabled + connected peer count) #25178 — expose p2p connectivity (enabled + connected peer count)
3. fix(slasher): do not file data-withholding offenses while peerless #25179 — do not file data-withholding offenses while peerless
4. fix(sequencer): skip proposing when node has no connected peers #25180 — skip proposing when the node has no connected peers
5. feat(node): report per-component health with p2p peer count on GET /status #25181 — report per-component health with p2p peer count on GET /status
6. fix(node): reject sendTx when node has no peers to propagate the tx #25182 — reject sendTx when the node has no peers to propagate the tx
7. feat(p2p): warn periodically while node has zero connected peers #25183 — warn periodically while the node has zero connected peers

[spalladino]

Superseded by #25185, which groups this stack's dependent layers (connectivity signal + slasher/proposer/health/sendTx gates) into one PR with a commit per concern, targeting merge-train/spartan-v5 directly. The independent fixes remain as #25177 and #25183.