OTEL `chat` spans do not have the proper parent span for subagents

Describe the bug

When OTEL is enabled and copilot uses the task tool to start a subagent, an invoke_agent span is created which contains all the execute_tool spans, but a single chat span at the very beginning. All other chat spans do not have the appropriate parent set to the invoke_agent span.

This is especially confusing when copilot starts multiple subagents, because then the chat spans are all mixed together, and the github.copilot.turn_id do not make sense.

Affected version

No response

Steps to reproduce the behavior

1. Enable OTEL
2. Ask copilot to use the task tool multiple times, e.g. "Using the task tool, ask three sub-agents to summarize the last five commits of this project, then compare their summaries."
3. Inspect the OTEL trace

Expected behavior

No response

Additional context

No response

Reproduisez la trace avec OTEL activé à l’aide de plusieurs sous-agents task-tool, puis inspectez les spans invoke_agent, execute_tool et chat obtenus. Suivez le chemin de parenté des spans et vérifiez que chaque span de chat est regroupé sous le span invoke_agent approprié et que les valeurs de github.copilot.turn_id ne sont plus mélangées entre les sous-agents.