Unable to spawn subagents with `mai-code-1-flash-picker` when the main agent model is `gpt-5.4` or `gpt-5.5` with `deferTools: never` config

### Describe the bug

# Summary

Spawning a subagent with the `mai-code-1-flash-picker` model appears to break only when `deferTools: never` is set for some MCP server configs.

At first this looked like a model-pairing issue between a main conversation running on `gpt-5.4` / `gpt-5.5` and a subagent using `mai-code-1-flash-picker`. After more testing, the more accurate trigger seems to be the newer `deferTools: never` MCP configuration rather than the subagent model alone.

# Environment

- GitHub Copilot CLI `1.0.64-1`
- Main model: reproduced from `gpt-5.4` / `gpt-5.5`
- Subagent model: `mai-code-1-flash-picker`
- Interface: GitHub Copilot CLI / agent tool flow
- Additional condition: `deferTools: never` enabled for some MCP configs

# Actual Result

When `deferTools: never` is enabled on affected MCP configs, spawning the subagent does not behave correctly. In my testing this showed up as a tool failure such as:

```text
✗ Execution failed: CAPIError: 400 Tool 'tool_search' is not supported with gpt-5. (Request ID: C57D:3931E0:32D4049:3A7FACE:6A367C5C)
```

In some cases the spawn path may also return no useful subagent output instead of a normal success/failure response.

### Steps to reproduce the behavior

1. Configure one or more MCP servers with `deferTools: never`.
2. Start a session using a main model such as `gpt-5.4` or `gpt-5.5`.
3. Try to spawn a subagent with `model: "mai-code-1-flash-picker"`.
4. Observe the failed or empty subagent execution behavior.

### Expected behavior

The subagent flow should either:

1. spawn successfully with `mai-code-1-flash-picker`, or
2. fail immediately with a clear validation or compatibility error that explains the unsupported configuration.

### Additional context

- Before isolating `deferTools: never`, this appeared to be a parent/subagent model compatibility issue.
- The newer finding is that the problematic behavior only occurs when `deferTools: never` is enabled for some MCP configs.
- That suggests the real issue is likely in tool resolution / tool exposure during subagent startup under this MCP configuration path.

thanks

Bắt đầu bằng cách tái hiện luồng công cụ agent của GitHub Copilot CLI với một máy chủ MCP được cấu hình bằng `deferTools: never`, một model chính `gpt-5.4` hoặc `gpt-5.5`, và `mai-code-1-flash-picker` làm subagent. Theo dõi đường dẫn khởi tạo subagent xung quanh việc phân giải và cung cấp tool; hoàn tất khi nó thành công hoặc trả về lỗi tương thích hoặc xác thực rõ ràng thay vì lỗi tool hoặc đầu ra trống được báo cáo.

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