[Copilot Chat] Subagent tree item / section cannot be collapsed after expansion

Does this issue occur when all extensions are disabled?: No (Requires GitHub Copilot extension)

- VS Code Version: `1.134.0`
- OS Version: Ubuntu `26.04 LTS`

Steps to Reproduce:

1. Open Copilot Chat and run a prompt that triggers a subagent execution.
2. Click on the subagent section header (e.g., `Subagent: `) to expand its detailed logs.
3. Click the header again to collapse the block.
4. Observe that the subagent block remains expanded and fails to collapse.

https://github.com/user-attachments/assets/e09c8bab-8d7f-4c10-b4d2-a452033cd907