Agent-created child sessions ignore a preset plugin-provided custom agent and fall back to Default agent

### Summary

When an **agent** creates a child session and presets its agent (via the session-creation tool that accepts an `agent` parameter), a **plugin-provided custom agent** is not applied. The new session silently starts on **Default agent** instead. Manually selecting the same agent from the session's agent dropdown works fine, so the agent itself is valid and loadable — only the agent-created "start this session in agent X" path fails for plugin-contributed agents.

### Environment

- GitHub Copilot app (agent-native desktop), Windows
- App version: `1.0.21`
- Custom agent provided by an **installed plugin** (appears as a selectable entry in the agent picker)

### Steps to reproduce

1. Install a plugin that contributes a user-invocable custom agent (the agent shows up in the agent dropdown).
2. Have an agent create a child session and set the preset agent to that plugin agent (the `agent` field on the session-creation / kickoff call).
3. Let the session start (with or without a kickoff prompt).
4. Open the created session and check its agent dropdown.

### Expected

The created session starts bound to the specified plugin-provided agent (dropdown shows that agent; the session runs under that agent's persona/workflow).

### Actual

The session starts on **Default agent**. The preset agent is silently ignored — no error or warning is surfaced. The kickoff prompt (if any) runs under the Default agent instead of the intended one.

### What I tried (identifier is not the problem)

I attempted the preset with three identifier forms; all three resulted in Default agent:

1. Bare agent name — ``
2. Plugin-qualified — `:`
3. File-basename-qualified — `:.agent`

I verified the identifier is correct:
- The plugin's `plugin.json` `name` matches the namespace used.
- The agent's frontmatter `name` matches the label shown in the agent picker.
- The agent is present and selectable in the UI dropdown, and **manually selecting it works**.

This points to the agent-created preset path not resolving/applying **plugin-contributed** agents (built-in/workspace custom agents may behave differently), rather than a wrong identifier.

### Impact

A workflow that has an agent spin up child sessions preset to a specific plugin agent can't work — every session must have its agent selected by hand in the UI, defeating unattended/parallel session orchestration. Silent fallback to Default also means automated runs can appear to "work" while actually executing under the wrong agent, producing misleading results.

Bắt đầu bằng cách lần theo tham số agent của lệnh gọi tạo session hoặc kickoff và so sánh đường dẫn phân giải của nó với đường dẫn của danh sách thả xuống agent, sử dụng các bước tái hiện với một agent do plugin cung cấp. Được xem là hoàn tất khi một child session được tạo bằng agent của plugin mở ra với agent đó được chọn và chạy kickoff prompt dưới agent đó, thay vì âm thầm quay về Default.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.