feat(playground): return finalized span at end of subscription

[axiomofjoy]

returns span on last subscription payload

subscription ChatCompletionSubscription($input: ChatCompletionInput!) {
  chatCompletion(input: $input) {
    ... on TextChunk {
      content
    }
    ... on ToolCallChunk {
      id
      function {
        name
        arguments
      }
    }
    ... on FinishedChatCompletion {
      span {
        id
      }
    }
  }
}

resolves #5037
resolves #5091
resolves #5037