feat(network): improve state sync peer registration and tracking

[muXxer]

Description of change

• Adds a StateSyncHandshake protocol — on connect, both sides immediately exchange chain identity (genesis checkpoint), sync height, and pruning watermark. This eliminates a race condition where push_checkpoint_summary could arrive before the peer was registered (the old flow required multiple sequential queries).
• Fixes PeerHeights insertion — new peers are now registered with complete info (chain identity + height + lowest watermark) in a single step, and the lowest field is correctly updated on reconnect.
• Filters disconnected peers in PeerBalancer — peers that disappeared between balancer construction and iteration are skipped, avoiding wasted requests. The RTT selection window is also widened (2 → 10).
• Periodic PeerHeights cleanup during sync — temp maps are pruned every 10k checkpoints to prevent unbounded memory growth during long syncs.
• Removes the legacy get_checkpoint_summary(Latest) fallback in query_peer_for_latest_info.
• Improves error context — failed checkpoint sync now reports connected/known peer counts.

Links to any relevant issues

fixes #11440
fixes #11441

How the change has been tested

• Basic tests (linting, compilation, formatting, unit/integration tests)
• Patch-specific tests (correctness, functionality coverage)
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes

Release Notes

• Protocol:
• Nodes (Validators and Full nodes): Hardened state sync peer tracking:
  • added a handshake protocol to eliminate race conditions during peer registration
  • fixed stale peer handling
  • added periodic cleanup to prevent memory growth during long syncs
• Indexer:
• JSON-RPC:
• GraphQL:
• CLI:
• Rust SDK:
• gRPC: