Fix flaky UtxoStateNodesSyncSpec: compare best-chain headers, not every known header

[Ergologica]

The problem

The Run it node tests CI job has been red on master since 4a7dba0 (2026-07-08), so every pull request currently shows a failing check. The failing assertion is always the same:

- should Utxo state nodes synchronisation (5 blocks) *** FAILED ***
  List("a369836d…", "9b7eb811…", "a369836d…", "9b7eb811…", "a369836d…", "a369836d…")
    did not contain only ("a369836d…") (UtxoStateNodesSyncSpec.scala:42)

Four nodes are queried, but six ids come back: two of them know two headers at that height.

Why it happens

/blocks/at/{height} returns every header id known at the given height, not only the one on the best chain. From the scaladoc of HeadersProcessor.headerIdsAtHeight:

single id if no forks on this height
multiple ids if there are forks at chosen height.
First id is always from the best headers chain.

The spec mines with three generating nodes and then compares headers.flatten, i.e. the union of every header each node has seen at that height, orphaned ones included. The checked height is initHeight + blocksQty - forkDepth and forkDepth == blocksQty, so it is the starting height — exactly where competing blocks are most likely while several miners race from the genesis block.

The result is that the test fails whenever a short fork occurred and was correctly resolved, i.e. on a perfectly synchronised network.

The neighbouring integration specs already handle this correctly: ForkResolutionSpec compares headerIdsByHeight(...).headOption and DeepRollBackSpec uses .head.

The fix

Compare the best-chain header of each node (the first returned id) instead of the flattened union, and assert explicitly that no node is missing a header at that height. This preserves the property the test is meant to verify — all nodes converge to the same chain — without failing on forks that were already resolved.

Test-only change, no production code touched.