Make command risk assessment explanations respect `github.copilot.chat.localeOverride`

The command risk assessment explanation shown in the terminal tool confirmation dialog should respect the `github.copilot.chat.localeOverride` setting.

For example, when the following setting is configured:

```json
{
"github.copilot.chat.localeOverride": "ja"
}
```

regular Copilot Chat responses are displayed in Japanese, but the AI-generated command risk assessment explanation is still displayed in English.

### Steps to reproduce

1. Set `github.copilot.chat.localeOverride` to `ja`.
2. Enable command risk assessment with `chat.tools.riskAssessment.enabled`.
3. Ask the agent to run a command that requires confirmation.
4. Open the terminal tool confirmation dialog.
5. Observe that the risk assessment explanation is displayed in English.

### Expected behavior

The risk assessment explanation should be generated in the language specified by `github.copilot.chat.localeOverride`.

Alternatively, a separate setting could be provided for selecting the language used by risk assessment explanations.

### Why this is important

Risk assessment explanations are safety-critical information intended to help users decide whether a command should be allowed.

Users who configure Copilot Chat to use their preferred language may not fully understand an English-only risk explanation, particularly when it describes destructive operations, remote state changes, package installation, or other security risks.

The risk level color and icon are useful, but users also need to understand the explanation describing what the command will actually do.

### Related issues

* #280512 — Slash commands do not respect `github.copilot.chat.localeOverride`
* #313991 — Terminal command risk assessment