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

启用 OTEL，使用多个 task-tool 子代理重现该 trace，然后检查生成的 invoke_agent、execute_tool 和 chat span。跟踪 span 的父级关系路径，并验证每个 chat span 都归组到相应的 invoke_agent span 下，同时 github.copilot.turn_id 值不再在子代理之间混杂。