fix(eve): Nest turns under the caller's propagated trace context

[Luca Forstner (lforst)]

• braintrustEveInstrumentation() records each Eve agent turn as an eve.turn span, with the model calls and tool calls of that turn as its children.
• Before, an eve.turn was always a root span unless it belonged to an Eve subagent.
  • So when a traced service started an Eve session and passed its W3C traceparent header, the turn still showed up as its own separate trace.
  • Following one request meant jumping between traces by eve.session_id. Online scoring rules on the caller's span also couldn't see the agent's tool calls.
• Eve already passes the caller's trace context to the instrumentation as parentTraceContext on its turn.started event, but the SDK ignored it.
• Now, when a turn has a parentTraceContext and isn't a subagent turn, the eve.turn span becomes a child of the caller's span, inside the caller's trace.
  • Model and tool spans of the turn, and the turns of any subagents it calls, stay in that same trace too.
  • This is the same mapping the SDK already uses for incoming traceparent headers: the trace id becomes the root and the caller's span id becomes the parent.
  • The link to the caller's trace is saved in the state Eve persists, so it also holds when Eve replays the turn in another process.
• Subagent turns are unchanged: they still nest under the tool call in the parent agent that started them.
• In Eve 0.47 and later, Eve only reads traceparent on session creates that include a remote-subagent callback. Plain eve/client sessions won't nest until Eve forwards traceparent for those requests as well.
• resolves Eve provider ignores parentTraceContext, so eve.turn is always a root span #2552