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

Riproduci la trace con OTEL abilitato utilizzando più subagenti task-tool, quindi ispeziona gli span invoke_agent, execute_tool e chat risultanti. Segui il percorso di parentela degli span e verifica che ogni span chat sia raggruppato sotto lo span invoke_agent appropriato e che i valori di github.copilot.turn_id non siano più mescolati tra i subagenti.