feat(ai-monitoring): Return conversation title behind apiVersion=2

[vgrozdanic]

Follow up to #120665, which added title to the conversations list and left the details endpoint alone because adding one there breaks its response shape.

That endpoint returns a bare array of spans, so there is nowhere to put a field that describes the conversation rather than a span. Passing apiVersion=2 now returns an object instead, with the spans moved under data:

// no apiVersion, or apiVersion=1 — unchanged
[ {span}, {span} ]

// apiVersion=2
{
  "conversationId": "abc",
  "title": "Refund a duplicate charge",  // null when nothing has been titled yet
  "data": [ {span}, {span} ]
}

Requests that don't send the parameter keep the old array, so this lands without touching the UI. The frontend switches over in its own PR, and only once it pins apiVersion=2 explicitly does flipping the default become safe for cached bundles. An unsupported value is a 400 rather than a silent fallback. Pagination is unaffected either way — cursors live in the Link header, not the body.