[Fix] Best block extraction id in separate field in /blocks/at/{height} API method response

[DSCmatter]

This Closes #978

This PR updates the /blocks/at/{height} endpoint to return a structured response that explicitly distinguishes the best block from forked blocks at a given height. This removes ambiguity in the API response and makes fork handling explicit for API consumers.

Changes

API Logic

• BlocksApiRoute.scala
  • Modified getHeaderIdsAtHeight to return:

    {
      "best": "blockId",
      "forks": ["forkId1", "forkId2"]
    }

BlocksApiRouteSpec.scala

• Updated the get block at height test to assert the presence and correctness of the best and forks fields.

OpenAPI Specification

• openapi.yaml
  • Updated the /blocks/at/{blockHeight} response schema to match the new structured format.