execution_subagent detached terminal completion does not notify parent chat

[GithubCopilot]

Does this issue occur when all extensions are disabled?: No. This flow requires GitHub Copilot Agent mode.

- VS Code Version: 1.134.0
- OS Version: Microsoft Windows 11 Enterprise 10.0.26200 (build 26200)

Steps to Reproduce:

1. In GitHub Copilot Agent mode, run a finite test command through `execution_subagent`.
2. Let the subagent reporting window expire while the test continues, causing the command to detach into a terminal.
3. Let the detached terminal finish successfully.
4. Wait without sending another chat message.

## Actual behavior

No completion event resumes or notifies the parent chat. The completed result becomes visible only after the user sends another message and terminal context is refreshed.

Observed instance:
- Detached terminal ID: `90c095d0-dafb-44cd-b24f-cebf1d5103cc`
- Exit code: `0`
- Test result: 2,689 passed, 0 failed, 7 skipped
- No parent-chat completion notification was generated.

## Expected behavior

A terminal detached by `execution_subagent` should remain owned by the parent agent. When it completes, VS Code should notify or resume the parent chat with the terminal ID, exit code, and final output, matching directly owned asynchronous terminal behavior.

## Impact

The agent can miss completed builds or tests, leave workflows unfinished, and require the user to prompt manually for status.