Agents window: BYOK custom-endpoint model not selectable for WSL workspaces

**Issue Type:** Bug Report

**VS Code version:** 1.134.0 (user setup)
**Copilot:** 1.0.81-0
**OS:** Windows 11, WSL2 (Remote-WSL, Ubuntu)

---

## Summary

A Bring-Your-Own-Key (BYOK) model configured via the Custom Endpoint provider appears in the Agents window model picker for **local Windows workspaces**, but is missing for **WSL workspaces**, even with the required experimental flag enabled.

## Settings (User scope, global `settings.json`)

```json
"chat.agentHost.enabled": true,
"chat.agentHost.byokModels.enabled": true
```

## Model config (`chatLanguageModels.json`, Custom Endpoint, Chat Completions)

```json
[
{
"name": "DeepSeek",
"vendor": "customendpoint",
"apiKey": "",
"apiType": "chat-completions",
"models": [
{
"id": "deepseek-v4-pro",
"name": "DeepSeek V4 Pro",
"url": "https://api.deepseek.com/chat/completions",
"toolCalling": true,
"vision": false,
"thinking": true,
"contextWindow": 1000000,
"maxOutputTokens": 384000
}
]
}
]
```

## Steps to reproduce

1. Add the BYOK Custom Endpoint model above via `Chat: Manage Language Models`.
2. Enable `chat.agentHost.enabled` and `chat.agentHost.byokModels.enabled` in **User** settings.
3. Fully restart VS Code (all windows closed).
4. Open a **WSL** folder, open the Agents window, start a session with the **Copilot** harness.
5. Open the model picker.

## Expected

The `DeepSeek V4 Pro` model is listed and selectable.

## Actual

The model is absent from the picker. It **does** appear in the Language Models editor (`Manage Language Models`), and it **does** appear in the Agents window picker when the session workspace is a **local Windows** folder instead of WSL.

## Additional context

- The same model works fine in the regular **Chat view** in WSL (selectable and functional, including agent/tool use).
- The gap is specific to the **Agents window + WSL workspace** combination for BYOK custom-endpoint models.
- The model has `toolCalling: true`, so it isn't being filtered out for lack of tool support.